Showing posts with label axiomatics. Show all posts
Showing posts with label axiomatics. Show all posts

Sunday, January 22, 2017

A simple recipe


Re Einstein’s relativity theory of 1905:

The new theory is based  in its entirety  on two postulates:
 1.  The laws of physics take the same form in all inertial frames.
 2. In any given inertial frame, the velocity of light is the same  whether the light be emitted by a body at rest  or by a body in uniform motion.
-- Abraham Pais, Subtle is the Lord (1982), p. 141

A simple recipe -- but which requires some care in the baking !


[For our own essay at minimalist postulationism, try:  

Sunday, June 12, 2016

Constructivist Angelology



But yet when considered, may help us to enlarge our thoughts  towards greater perfections of it  in superior ranks of spirits. … The several degrees of angels  may probably have larger views.
-- John Locke, An Essay Concerning Human Understanding (1690)



Man’s understanding, though allied to the angelical, operates differently.  The angels understand intuitively, man by the painful use of the discursive reason.
-- E. Tillyard, The Elizabethan World Picture (1942)

It is presumably not obvious to the chimpanzee (or, if this be setting his smarts too low, to the humble woodchuck) that for all m, n in Z, m + n = n + m.  Nevertheless, in his daily scurryings and burrowings, he will repeatedly meet up with particular instantiations of this modest truth.
            For the woodchuck (at any event the southern northeastern lesser striped variety) builds a number of nests and other temporary dwellings, each of which has the framework of a variously triangulated  polyhedron, built tinkertoy-fashion from a fixed number of sticks.  Now, gathering them one by one would take too long, nor can the tidy woodchuck stand to have any sticks left over.  So when constructing his summer dwelling -- an icosahedron, which needs thirty sticks (did I get that right? My calculating powers are not much beyond those of a woodchuck) -- he normally harvests a jubjub bush, which has twenty-two sticks of exactly the right specs and which blooms in the spring, then rounds it out with the eight-sticked glubglub bush, which sprouts slightly later. 
But then one year, the blooming of the jubjub was delayed, and the woodchucks despaired.  All but one, the enterprising Willie, who went doggedly (or groundhoggishly) ahead  and harvested the available glubglub, supplementing this  when the jubjub arrived slightly later.  This remarkable exploit was recorded in the annals: for 22 then 8, one may substitute 8 then 22.
            It was subsequently found that a mubmub bush (18 sticks) followed by a nubnub bush (12) would do just as well – und zwar, in either order!  This fact too was recorded.
            The years went by, then the centuries, and the millennia, and the annals grew to seven times seventy stout volumes, densely filled with such arcana as: a cube-for-cubs may be constructed of a lublub (7) plus a rubrub (5), and this in either order; and so on for billions of examples.  All this was considered a branch of botany, a purely empirical science.
            By this means, the woodchucks arrived at an analogue of Babylonian mathematics.

Interlude:   A physicist depicts the arithmetical state-of-play in a papyrus from Egyptian/Babylonian times:

It records the resolution of a great number of fractions  into a sum of aliquot parts,  the original numerator always being 2:  as, for instance,

2/97 = 1/56 + 1/679 + 1/776

But no rules are given for effecting such resolutions, and the whole treatise seems to be a mere compendium of results obtained by repeated trials.
-- James Jeans, The Growth of Physical Science (1947 [posthum.]; 2nd edn. 1951), p. 11

            Until one day one Wisedome Woodchuck, a distant descendant of Willie, figured the whole thing out, and in a remarkable demonstration of only eighty pages (rather hard to follow, but sound), showed that m + n = n + m  was a perfectly general fact, replacing the seven-times-seventy volumes at a stroke, and freeing up his brethren for yet further architectural innovations, which previously had been shunned, as their particulars were not yet in the book.  The annals were placed in a museum, which the elder woodchucks might still visit, marveling at favorite exhibits (as who could forget that remarkable winter, when 5,878 + 519 turned out to be equal to 519 + 5,878?  A tour de force!). Meanwhile generations of young woodchucks (the pride and despair of their parents, who could not follow them into Canaan, with their aging brains) studied Wisedome’s proof, breaking their little heads against it.

           
Meanwhile in Metropolis… The humans, learning of this, politely saluted Wisedome’s modest accomplishment, and experienced a pang of sympathy for woodchuck-kind; yet felt no inclination to visit their Museum of Particular Results: for which they felt, indeed, a kind of horror.  And even the general result, while true, is somehow to us not truly interesting. In any case we are all too busy wrestling with the Riemann Hypothesis, to have time to look back.

Meanwhile in Elysium, where throne the angels sensu strictior, the lowest order of angelic beings sensu lato, a mock compliment is paid to Andrew Wiles, who finally figured out that little Fermat puzzle, with which the angel-kind  are wont to amuse the nursery.  Not that the angels arrived earlier at his proof, nor any refinement thereof.  They simply scoop up a few infinities of integers with their fractal fingers, twist them this way and that—and see, it doesn’t fit!  Simple.
            Moreover, all facts about all structures of ordinal type omega, whether or not deducible by any finite axiomatization, are equally transparent to the angels. They just look.

            So, is Elysium the mathematical Paradise?  Not quite…

            In a remarkably lucid and accessible article*, which should be packed into every pupil’s lunchbox by a considerate mom, Gödel observes that our continuing failure to resolve Cantor’s continuum problem, left over from the previous century, is quite an embarrassment.  It means that we are unable to wrap our minds around the very simplest multiplication problem possible, beyond the finite ones that these days can scarcely stump a woodchuck. Namely, two times two (times two, times two – keep going).  He writes:
            “It is easily proved that the power of the continuum is equal to 2^(aleph-nought). So the continuum problem turns out to be a question from the ‘multiplication table’ of cardinal numbers: namely, the problem of evaluating a certain infinite product (in fact the simplest non-trivial one that can be formed).  There is, however, not one infinite product (of factors > 1) for which so much as an upper bound for its value can be assigned. […] It is not even known whether or not m < n implies 2^m < 2^n.” 
            We are  so to speak  staring helplessly  at a pile of sticks.

            Nor does the subsequent Cantor+Cohen demonstration of the independence of the continuum hypothesis  from a particular system of axioms for set theory   set the matter aside. Gödel had already anticipated Cohen’s result, and wrote:

A proof of the undecidability of Cantor’s conjecture from the accepted axioms of set theory (in contradistinction, e.g., to the proof of the transcendency of pi) would by no means solve the problem.  For if the meanings of the primitive terms of set theory … are accepted as sound, it follows that the set-theoretical concepts and theorems describe some well-determined reality, in which Cantor’s conjecture must either be true or false.

            Indeed Gödel suspects that the Cantor conjecture is actually, factually false: which means that somewhere, among the actual literal real numbers, there is hiding a set of cardinality intermediate between aleph-nought and its power set, with definite members which the angels could name.  Not, however, the lowest order thereof; this lies beyond them.  But at the next step up, the archangels hang these sets from mobiles over their infants’ cribs.  In fact a woodchuck may somewhere inadvertantly have used one of these sets for nesting materials, and even now lies sleeping on it – a night of troubled dreams.

            So much for a simple pancake-stack of omega-many deuces – the limit of the lower-angels’ ken.  What about the square root of omega-to-the-omega; or cross sections of fibre bundles on toroidal cap-omega-cross-theta space? For each level of angels, there will be something beyond them that they just don’t get.

*

There are two poles of the range of approaches to the problem of infinities.  One is that of the badger-like Brouwer, who simply sweeps the chessmen to the floor, folds up the board and goes home.  (An only somewhat more amenable figure, says Gödel, is Weyl, who allows as how there might be something to board games, but suggests we play checkers – or Chutes ‘n Ladders – rather than chess.)  The other pole says:  Infinities are tricky, but they all exist, and are present to the Infinite Mind. Gödel himself uses that term, e.g. noting that Ramsey’s admission of formulae of (countably) infinite length  might be constructivistic for an infinite mind  but not for our own.  Gödel does not, however, seem to feel much need for any desperate appeal to such a mind, in the course of an ordinary day, since he -- like Badger’s amiable friend the Water-Rat-- is a thoroughgoing Realist, and comfortable as such in his own skin.  For him the assumption of infinite classes “is quite as legitimate as the assumption of physical bodies, and there is quite as much reason to believe in their existence.”  The outwardly gloomy Austrian  is really the jolly Dr. Johnson of set theory.
            Only now there’s a problem, of a sort which did not confront the schoolmen, who never counted on the uncountable:  the Infinite Mind is all very well, but -- Which infinity did you have in mind?
            Who comprehends *everything*? God does, by definition. Yet He cannot be simply the crown on a tower of constructively ascending intelligences.  He is like an “inaccessible cardinal” – and not the first.  Nor perhaps ‘the last’, if there is no last.  Whatever He might be, there is Cantor in the wings, grinning, waiting to perform a Power Set on God, yielding – what?  -- Nothing one can begin to commence to pretend that we can approach with our sadly finite understanding.

            All of which suggests, if nothing else does,  that God is something more and other than an alternately wrathful and affectionate granddad  with a perfectly enormous white beard – however much longer that beard might be, than the stubble which disfigures your chin or mine.  Who one day, apparently from sheer idleness, as one might choose chocolate, chose the Jews.  Who later, some say, cast a Jove-like eye  on a certain Palestinian virgin.  And who at present is very angry indeed with the Democrats (or the Ravens, or whomever).  Yet what He in fact might be, we cannot even begin to imagine anyone’s beginning to conceive.  (Cf. the suggestion of 1 Kings 8:27  that the heavens themselves have heavens (and so on up); and that the whole omega-tower of them  cannot encompass God.)

*     *     *
~ Commercial break ~
We now return you to your regularly scheduled essay.

*     *     *
            We actually wind up with a sort of hamstringing of the Ontological Argument. Notoriously its conclusion does not really follow from its premise;  but now even its premise limps: “Since we can imagine a Perfect Being…”  But that’s just it, we can’t!  Not even little infinite bits of one! Yet paradoxically (and God reportedly loves paradox – at least Chesterton does, His publicity agent on Earth), this seeming stomping on the prostrate corpse of the offspring of Anselm, this despairing cry that somehow even Infinity does not suffice, so far from opening the agora  to legions of snickering atheists chanting “Toleja so!”, points somehow upward, -- outward,   -- onward ….  Praise Him!


Postscript:
John Locke himself, normally regarded as the Poster Boy for Empiricism, of I'm-from-Missouri common-sensicality, yet delivers himself of this (Essay, III.vi.12):
That there should be more species of intelligent creatures above us, than there are of sensible and material below us, is probable to me from hence:  that in all the visible corporeal world, we see no chasms, or gaps.

That is to say:  The gap between ourselves, and God, must somehow be filled, according to the Principle of Plenitude.


And again (IV.iii.23):

He that will consider the infinite power … of the Creator of all things, will find reason to think, it was not all laid out upon so inconsiderable, mean, and impotent a creature, as he will find man to be;  who  in all probability, is one of the lowest of all intellectual beings …
Angels of all sorts are naturally beyond our discovery, and all those intelligences, whereof ‘tis likely there are more orders than of corporeal substances, are things, whereof our natural faculties give us no certain account at all.

Since theism is far from central to Locke’s Essay, it is curious to see the emphasis on this scala naturae idea.

--------------
*”What is Cantor’s Continuum Problem?”, repr. Benacerraf & Putnam, eds., Philosophy of Mathematics.

~

Postscript:  For the possibility that the structure of certain mathematical truths relating to an infinite domain  might resist any but a case-by-case “Babylonian” approach, cf. the quotation from Michael Dummett towards the end of this post:


Compare further (re ascending ranks of abstraction and generality):


.


Sunday, January 31, 2016

On What There Is (expanded)


Existence is -- what existential quantification expresses.
       -- W.V.O. Quine, “Existence and Quantification”  (epigrammatic punctuation added) 


And, contra a couple of celebrated slogans of Quine:

The locution ‘ontological commitment’ is not one I have any use for, and neither do I care to ask or to answer the curious question “What is there?”  I say “There are chairs in the room,” and if someone wants to say “Therefore there are chairs”, tout court, it sounds odd… If this is ontology, then ontology is a mouthful of air.
-- Paul Ziff, Semantic Analysis (1960)


The subject of this essay is ontology; we gave a foretaste of the subject here.


By its dictionary definition, ontology is the study of Being.   Now, for me, “What is Being?” is the ultimate conversation-stopper;  that question, like Being itself in so bald an encounter, is like a diffuse and vaguely repugnant blancmange, filling all space.   It is questions like that which persuaded me early on that I was not interested in Philosophy.   And at that level, I still am not.



Quine, it turns out, is of like mind, for he remarks, of his epigram above, “This is as unhelpful as it is undebatable, since it is how one explains the symbolic notation of quantification to begin with.  The fact is that it is unreasonable to ask for an explication of existence in simpler terms. … Explication of general existence  is a forlorn cause.”

(Similarly, this, self-stultified by its own generality:

      The meaning of a remark in any language.
-- section-heading in: Jonathan Cohen, The Diversity of Meaning (1963), p. 154 )

A pre-philosophical, psychosociological observation:  The opportunity of waffling-on about Being with a capital B, seems to bring out the worst in writers.  As, Emerson (well, not quite fair to ontologists, since it doesn’t take much to bring out the worst in Emerson -- an ordinary pen-nib will do), in his celebrated essay “Compensation” (1841):

There is a deeper fact in the soul than compensation, to wit, its own nature.  The sould is not a compensation, but a life.  The soul  is.   ….  Being is the vast affirmative, excluding negation, self-balanced, and swallowing up all relations, parts and times  within itself.

Now that is ten pounds of horse-doody  in a five-pound bag.
~

At only one level down of abstraction, philosophers have traditionally brooded upon the ontological status of qualities or attributes or essences -- “whether concepts have a supramundane, or only a psychological existence;  whether they are transcendent intuitables or only private instrospectibles.” (Gilbert Ryle, “Ordinary Language” (1953).)   For hardcore Realists like Meinong and even early Russell, “Consistently with the assumed equation of signifying with naming, they maintained the objective existence of all sorts of abstract and fictional entia rationis.” (id., “The Theory of Meaning” (1957).)


The question becomes more compelling in the context of the philosophy of quantification (“To be is to be the value of a variable,” quoth the Quine); and shapelier still in the context of what counts as an ‘item’ for, say, physics.
(Incidentally… I have borrowed the title of this post from a well-known work (1948) of the eminent Harvard philosopher, who taught me logic when I was but a wee lad.  Quine, having passed to a different realm of quantification, will doubtless not object.)
(Sudden update:  Just browsing around, I notice that this is actually the second time I stole  the title -- Shakespearian in its simplicity -- of my former magister;  earlier effort here.)

Thus, the focus shall be now  not on Being, but on beings -- on what counts, for us, as entities, when we pursue science, and why.   As Quine nicely puts it:  “Ontology … is a generalization of somatology.” (Roots of Reference (1973), p. 88).  The top-down approach to ontology -- “What is Being?” -- is baffling;  but starting from what we understand, we might build upwards.

Thus, we confront more tractable matters of hypostasis (reification) and individuation.  
By getting down in the weeds concerning what choices have been hit upon by the various scientific enterprises that have had to deal practically with such matters, we might in time return refreshed to the general question.

Consider this analogy.  The ancient Greeks asked themselves, “What is motion?”, and discovered that, once you dig into the matter, it’s more puzzling than it looks  -- cf. St. Augustine’s celebrated quip about the meaning of Time:  If you don’t pose the question, I know perfectly well;  if you ask me point-blank, I am flummoxed.  Some philosophers even came to feel that the very notion of motion was paradoxical, or impossible : compare more contemporary thinkers with similar doubts about Free Will.  (Eppur’, in both cases, si muove.)  Once one has studied the matter, however, in classical dynamics and in special relativity, and understood how (Achilles and the tortoise) an infinite series may yet sum to a finite value, you return to the matter with new confidence.

~     ~     ~

We must concede at the outset that ontological quandaries seldom arise in daily life.  Only very occasionally, and that not systematically, do you pose What-There-Is questions.  Things like:  Does Bigfoot exist?  Does Dark Energy?  (Everyday life if you’re a physicist, that is.)   True, a questing undergraduate may once in a while trouble himself with questions such as the Existence of Other Minds, and Is the Universe an Illusion;  but such queries cease once he gets himself a proper girlfriend.
 
~     ~     ~

The atomists, in their purest and here somewhat idealized form, imagined a world in which indivisible particles were the basic Things, all else being combinations of these, and thus, in the most parsimonious view, ontologically subaltern.   (Leibniz imagined something rather like this for the noösphere, with his ineffable monads.)  And indeed, we can well imagine a world, in which such entities entered into but fleeting congeries, without definite or lasting outline, and crucially, with no emergent properties for the ensembles (thus, in particular, no reproduction of atomic ‘clouds’).   Such a world would have an essentially unambiguous, monolevel ontology.


Now, however, consider a different world:  a pool table.  And -- for this is necessary too, and we rather finessed the question in the fable immediately above -- consider that we have been given a task:  viz, to characterize the perambulations of matter atop it.   In this scenario, it is the billiard balls themselves we must consider, and in no wise the atoms that constitute these.
 
Consider now Euclidean geometry.  Here, fundamental ontological status was posited for just two entities:  the point, and the line.   (Notoriously,  one can present this geometry in a ‘substrate-neutral’ way that professes agnosticism as to the nature of these posited ‘points’ and ‘lines’ -- beer-mugs and beer-mats, we could call them just as well.  And, more tellingly, styles of geometry in which the point and the line are dual to each other, thus interchangeable.  But to consider this further, were to sail afield.)
No higher figures were distinguished as fundamental -- neither the triangle nor the ten-million-and-seventeen-gon  enjoy axiomatic status as part of the furniture of the Euclidean universe.  And indeed, in the broader perspective (the Erlangen Program) which sorts out and makes sense of a variety of geometries, it is not the individual figures  on which all things hinge, but their transformations -- their symmetries, and the way these form an algebraic Group.

~     ~     ~

The prototypical example of an indisputably extant entity is you.  You are physically coherent, you have purposes and plans, you are self-aware from moment to moment; ontologically, it doesn’t get better than this.  And if you’re Donald Trump, you’re done:  end of ontology.  You slide through life like a bubble down the duodenum, a blob of solipsism.
For the rest of us, we embrace the existence of Other Minds, and indeed quite on a par with our own.  
And now comes (as Blessed Pope John Paul II put it ) an ontic discontinuity or  “ontological gap” between ourselves and the beasts.   There is a spiritual truth to this, but biologically, it does not cut nature at the joints.

(Note:  That gap itself should not be over-emphasized, since, in the grand mediaeval vision of the scala naturae, it is just one of several such.  Roughly:
archangels -- seraphim -- cherubim -- penguins -- mankind -- critters -- Protista -- sludge.)


[Click that image for more exciting details!]

So we admit the biosphere -- only, just where to draw the lines among individuals gets murky, the more you learn about what-all is out there.  Herd animals, species all of whose members are genetically identical, parasites, incorporated former parasites such as plasmids and mitochondria, slime mold, elm forests (one giant subterraneanly-connected plant), and even such exotica as the cast-off arm of a male cuttlefish:  as Darwin put it, “So completely does the cast-off arm resemble a separate animal, that it was described by Cuvier as a parasitic worm”.
There is no fact-of-the-matter about such cases; their intershadings show that our question, Which are the functional individuals?, must be more sharply posed.



~
~  Posthumous Endorsement ~
"Were I alive today, and in the mood for a mystery,
this is what I would be reading: "
(I am Quine, the great and powerful;
and I approved this message.)
~         ~


Let us revisit the examples of the pool table, and the geometries.   Here the basic entities were identified relative to certain transformations of the roster of potential entities:  the billiard balls caroming about, rebounding, never blending, proved to be the units to reckon with here.   And in modern mathematics, the symmetry transformations of the individual geometries proved more important that the various squiggles and shapes (or collections of squiggles and shapes) that undergo them.    So perhaps the way forward is to consider the kinematics of life. Ecology, that is, and Evolution.


When the theory of Natural Selection was introduced to the world in 1859, species rose to prominence in our conception of the way the world really is, right in the title of that great work, The Origin of Species.  Individuation can be problematic when we contemplate such things as animals undergoing complete metamorphosis, sessile vs. vagile stages,  and so forth:  but at each moment the species are (in the somewhat idealized classic view) sharp in outline, non-interbreeding, reliable entities.  (From a NeoPlatonist perspective, the species may even be more real than any of the variously imperfect and misshapen individuals that instantiate that ideal.)  For a time, Nature red in tooth and claw was conceived as a battle among these  supra-individual entities, competing, going extinct -- tyrannosaur versus triceratops. predator and prey, the early mammals peering out discretely from the prehistoric underbrush, waiting their chance.


Yet no sooner had we managed to wrap our heads around the notion of the species   as the fundamental unit of biological accounting (which in particular, delightfully,  cleared up the mystery of sex), than a pot of cold water was flung in our face:

Why should a female  produce offspring carrying only half her genes, when by parthenogenesis … she could produce clones…?  The simple answer, that the variability produced by sexual recombination makes for greater adaptability, and is therefore ‘for the good of the species’, will not serve.  Darwinian natural selection … has to do … with individuals,  and selection for group characteristics  has no simple place.
John Bonner & Robert May, introduction (1981) to a reprint of Darwin’s Descent of Man.


The next step (and the consensus of current thinking) settles neither on individuals nor on groups, but on a unit which, in Darwin’s day, was not even known specifically to exist:  the gene.  The argument has been superbly laid out for the general public in Richard Dawkins The Selfish Gene, so we needn’t walk through the reasoning here.  The upshot is as follows:
Richard Dawkins, The Selfish Gene (1976; 2nd edn. 1989), p. 34:

In sexually reproducing species, the individual is too large and too temporary a genetic unit  to qualify as a significant unit of natural selection.  The group of individuals is an even larger unit.  Genetically speaking, individuals and groups are like clouds in the sky or dust-storms in the desert.  They are temporary aggregations or federations.

(This reminded me curiously of a suggestive passage from a historical-espionage novel by Tim Powers, Declare:
You know what the djinn tend to be made of, from moment to moment -- wind, dust, snow, sand, agitated water, swarms of bugs, hysterical mobs. )

Anyhow, Dawkins goes on to make clear that his definition is functional not anatomical:

The largest practical unit of natural selection -- the gene -- will usually be found to lie somewhere on the scale between cistron and chromosome.

This functional/structural rather than physical definition  is reminiscent of the notion of phoneme, as opposed to a phone or sound.

Edward Wilson concurs:

The average differences between people in different localities … are narrowing.  Genetic homogenization has similarities to the stirring together of liquid ingredients.  … But the most elemental units, the genes, remain unperturbed.  They stay about the same  in both kind and relative abundance.
-- E.O. Wilson, Consilience (1998), p. 273

Now we feel we are back on familiar ground.  These genes are rather like biological analogs of atoms, in the old Greek well-behaved, billiard-ball-like conception of these.  They just take some getting used to.


Yet even after the first ontological question has been answered (What is there?) in favor of the gene, there is still the second (What is it?).  As.

Should we think of a gene … as a structure that is replicated, or as information that is copied and translated?
J. Maynard Smith & E. Szathmáry, The Origins of Life (1999), p. 10


Actually, Dawkins makes a much simpler and apparently unanswerable argument for thus privileging the gene as a unit of accounting:

The true unit of natural selection has to be a unit of which you can say it has a frequency.


(Individuals and groupings obviously don’t fit the bill.)   This argument is completely general, and is independent of the details of biology.   Thus in particular, it should apply (if it is valid) mutatis mutandis  outside of biology.
For the style of thought, though not the detailed content, cf. Quine (“On What There Is”), maintaining that quantification is “the only way we can involve ourselves in ontological commitments”.


Further, compare this:
Gerd Gigerenzer et al, The Empire of Chance (1989), p. 246, quoting Read Tuddenham:
To the statistician's dictum that whatever exists can be measured, the factorist had added that whatever can be 'measured' must exist 


[For a brief and untendentious survey of the various candidates for status as a Unit of Selection, click here.]
~     ~     ~

Having persuaded us that the gene, rather than the individual or the herd or the species, is the fundamental reckoning-unit of life, Dawkins then complicates matters in a way reminiscent of those extended and ill-individuated entities like elm forests and slime molds, or even Bertrand Russel’s definition of the number ‘four’ as the set of all foursomes:

What is the selfish gene?  It is not just one single physical bit of DNA, it is all replicas of a particular bit of DNA.  … ‘It’ is a distributed agency, existing in many different individuals at once.

By this time it is clear that, the more you look into it, the ontology of biology looks more like biology and less like Ontology -- in that original maximally abstract metaphysical program to whose allurements we confessed ourselves deaf.

Still, this business of the gene, defined as a functional rather than a spatiotemporal unit, does get us back to old-fashioned ontology as practiced by philosophers.   Quine sparkles at this.   He wastes no time on “What is the Nature of Being?”, but rather rolls up his sleeves, and, in the chapter “The Ontogenesis of Reference”, constructs a plausible, insightful, and wittily-told fable of how we acquire our notions of objects, and what it is that we acquire.   (A wry tribute to the style of mind involved in such exercises  can be appreciated here.)   By page 98 of Word & Object (1960), he has made a case for the ontological respectability of “a single sprawling object”, and admonishes:

There is no reason to boggle at water as a single though scattered object, the aqueous part of the world.  Even the tightest object, short of an elementary particle, has a scattered substructure  when the physical facts are in.

To which, Amen;  adding only that, when even more surprising physical facts are in, concerning indistinguishable elementary particles (bosons, at any rate), there is a sense in which these too could be considered a single scattered object.


The genes (or bosons), as thus conceived, are only, so to speak, accidentally scattered;  things empirically might have been otherwise.  Consider now rather entities that are scattered by construction, by definition:  higher-level entities, sets or collections of lower ones.

Questions about the ontological status of such things can arise even in the everyday pre-philosophical world.  In what way can we say that the following are genuine entities, with lasting contours and cross-temporal identification, despite the changing roster of the individuals that make them up? -- Your (nuclear/extended/….) family; the Boy Scouts; the nation-state to which you belong.   This is a moral and practical matter, not simply ontological:  having pledged allegiance to any one of these at t=0, are we likewise bound at a later time, despite their ever-shifting membership (and foreign policy)?  (I address such questions in a projected essay, “Continuity of Identity”.)
So, we have noticed an actual ontological question within the cares of daily life.  Still, it is not to a metaphysician that you would turn for clarification, should your eighth cousin thrice removed suddenly show up on your doorstep, claiming ties of kin that give him the right to move in with you and to borrow your car,  nor to an ontologist, were the Boy Scouts ever to get the Bomb.

In Biology -- the fons et origo of structured higher-level objects in scientific practice -- such entities include:  species (made up of conspecific individuals);  genus (made up of species); family (made up of genera); order; class; and so on up.  Here the ‘atom’ is the individual animal or plant;  there is no place in the traditional taxonomy of considering an individual as a congeries of genes -- and indeed the set-theoretical structure is completely different, the gene-sets in question being radically non-disjoint, whereas an animal is either in one species or another, not both.
 

In the nature of the case, it is clear that the higher taxa of biology are not ontologically given as such, but are confections of convenience, based  to be sure  upon what’s out there, and proceeding via sound and defensible principles.  Thus in particular, whereas a species as a whole does pretty much hang together or hang separately (say, in a sexually reproducing species, if the survivors are too sparsely scattered to hook up), there is no such selective linkage among the various n-level groupings in a taxon at level n+1.  Should the echidna ever bite the dust, ‘twill be a sad day for all lovers of monotremes; but the valiant platypus  still will soldier on.
There have also been major revisions in higher-order taxa;  even some quite familiar ones (reptiles, insectivores, puffballs) have  upon closer inspection  been dismissed as polyphyletic.


~     ~     ~

So much for the entities of biology.  What of Chemistry -- which is “the next level down” in terms of the agenda of Consilience?

Here we are in for a pleasant surprise.  No such agonizing and backtracking will be necessary as it was before.  The answer is:  atoms.   And not just atoms, in a row as it were, but, stacked, structurally stacked, in a most revealing way.  This is the Periodic Table of the Elements, first unveiled to a grateful world by Mendeleev, of blessed memory.  It is possibly the single most satisfactory scientific object on the planet.  Moreover its elements and its structure reach directly, consiliently, straight down to basic physics.  It is a wonder to behold.

There is even a loose analogy between atoms-and-molecules, on the one hand, and genes-and-individuals, on the other.  Loose, but better than most of those cited by Wilson in his ambitious book.
~     ~     ~

Physics, by contrast, is in no such happy case.  Such subjects as cosmology or thermodynamics or hydrodynamics don’t seem to have ‘basic-level objects’ in any obvious way.   There are, to be sure, the “elementary” particles, but these have been as troublesome as they are helpful, referred to distastefully as the “particle zoo”.   What with quarks and various subtle symmetries, these have now been regimented into something more satisfactory, though still nothing like as self-explanatory as the Periodic Chart.  Further, they do not span the whole of physics, but only of Particle Physics, a subfield.


There are, nonetheless, deep ontological questions within physics, with still-tentative but sophisticated answers.   I am not currently competent to comment on these, but a selection of intriguing quotations may be consulted here.

~     ~     ~


Astronomy affords relatively little by way of ontological interest; but consider this wise observation by astronomer  Mike Brown (quoted in The New Yorker for 24 July 2006):

Planets are like continents. ‘Continent’ is a good geological word, but, like ‘planet’, it has no scientific meaning whatsoever.

That is an epigram, and thus permits itself a breezy way with words; meaning here really means ‘ontological status’.
The point is of course lost on the layman;  witness the heavy coverage of Pluto’s “dethronement” by the latest Kuiper-belt detritus, as though this were of the least importance for the understanding of our cosmos.  But far more important, the opposite assumption seriously misled some of the finest minds of the Middle Ages.  For Galileo’s misadventure with circular planetary orbits, click here.  For Kepler’s fine failed vision of the planetary distances reflecting nested Platonic solids, here.  Their basic insights were sound, even brilliant; but planets (i.e., floating lumps of dirt) simply don’t have the ontological status to deserve such angelical constructions.


~     ~     ~

In Mathematics, the conundrum concerns, not so much the existence of thís (class of) object versus that (class of) object, let alone which are the ‘basic-level’ objects (I know of none), but the existence of any objects überhaupt.  That is, we have retreated from the question of beings, and are back at the bad old topic of Being.   At best:  for in fact, the question is probably not best posed in terms of “the existence of mathematical objects”, which threatens to involve us in fruitless discussions of what they are exactly (e.g. the integers as really sets of one sort or another, including Russell’s extravagant suggestion), whereas in fact,  mathematical objects or entities or thingums or whatever they are, are the very plume and prototype of substrate-neutrality;  a less contentious formulation would be “the transcendence (epistemological independence) of mathematical truths”.  (One is less likely to wonder whether a “truth” is, say, pink, than whether an object is.)

Let us consider a specific question, with an at least superficially ontological aspect, that is more localized than that vast barely-answerable question about the ontological status of mathematics as a whole.  (My attempt at a Realist answer to that one begins here.)
For example:  Does there exist a topological object of the following description:  It is regular, second-countable, yet could never be assigned a metric?   Urysohn looked into the matter, and concluded that none exist.  But it wasn’t by looking around, or by exhaustive search, that he reached this conclusion, the way you might drag every inch of Loch Ness and finally conclude that it contains no monster.  Never quitting his armchair, he deduced the result, in a way in which things were never really serially considered.
Indeed we had to strain a bit to cast the problem in the form of a question about ‘existence’ at all:  it’s not like finding Bigfoot, or failing to find him.   If biology were like mathematics, then we could infer the existence or non-existence of Bigfoot, without ever actually spotting him, nor searching the wooded hills, based upon abstract patterns elsewhere in the system.  This is one of the very many ways in which biology and mathematics are not the least bit alike (I mention this only because of the counter-program of consilience - a nice idea, but a will-o’-the-wisp.)


Mathematics does nonetheless afford good grist for the ontology-mill, indeed more clearly ontological than anything we have yet seen.  Namely, the entities posited by what are known as “existence proofs”.    There is no properly (intra)mathematical doubt about these purported objects -- they uncontroversially have such&such properties, if indeed they are there to bear properties at all.  The problem is with the special sort of purported demonstration that says, although we may never see such a thing, yea verily, it doth exist.  Such proofs can be purely deductive, non-constructive; so that, although we are assured of the existence of something fitting a given description, we are given no hint as to how to find the item in question.  Understandable ontological qualms about such spectral beings led to the founding of a school of mathematics that rejects such non-constructive proofs:  Intuitionism.  This dog-in-the-manger school gets vastly less play in actual day-to-day mathematical practice, than it does in philosophy books.

The one place within mathematics where ontology is definitely at home is Set theory.  A typical credo:

I have written this book from an uncompromisingly realist or platonist position; that is, I have taken the viewpoint that  in some sense  sets do exist,  as objects to be studied, and that set theory is just as much about fixed objects as is number theory.
Frank Drake, Set Theory (1974), p. 18

Indeed, this subject is often practiced by ontologically-inclined philosophers (such as Quine) and taught in the philosophy department.  (That other mathematical outlier -- logic -- is likewise often so housed.  I took Intro Logic -- “Phil 140” -- from Quine.)  It is from this milieu that we got the slogan “To be is to be the value of a variable.”
Quine’s quip, suitable for recital to the babe in the cradle, is actually trickier than it sounds, owing to his notion of substitutional quantification, which does not express existence, vs. objectual quantification, which does.  There is a grey area of entities which, like most nonalgebraic real numbers, are assumed to lead just as robust an existence as the algebraic irrationals, but which are not finitely specificable.
(Further remarks on the ontology of logic and set theory  here.)
Note, incidentally, a certain resonance between this last distinction, and the notion in physics of observables -- an attempt to get a firm handle on What There (Really) Is, amid the welter of mathematical abstractions.
 
~     ~     ~

Linguistics and Anthropology come each in two flavors: on the one hand, traditional mostly-European philology and Völkerkunde; and on the other, a present-day, typically American scientistical approach.  The former fall under the Humanties.  They were not much concerned with positing abstract analytical entities;  the “parts of speech” go back to ancient times, and were defined intuitively, largely morphologically, which is something you can get away with in the highly inflected classical languages like Latin or Sanskrit or Greek.   The latter, by contrast, strives (or, in the case of Anthropology, strove; now it strives only to be politically correct) to be honest-to-goodness sciences like physics or anything else.   Many intricate and closely-argued entities were posited and fought over, for phonology and syntax (some in morphology and semantics too, of course, but those weren’t worth fighting over);  and anthropology became algebraically structural in its analysis of kinship systems.  Both fields were self-aware of what they were up to, and there was a running discussion of the ontology of the theories, under the genial rubric “God’s Truth vs. Hocus-Pocus”.  The God’s Truth faction took a Realist stance towards the posited analytic entities; the Hocus Pocus faction, a Nominalist.


~     ~     ~

Somewhat surprisingly, the study of Folklore is also distinguished by the positing of abstract analytic entities, known as motifs; and this, already in the early years of the twentieth century.  These were carefully and exhaustively catalogued in the Stith-Thompson Motif Index.  Their combinatorics determine the tale-types around the world.   They are reminiscent, not really of atoms (since the characteristics of molecules are so wildly ‘emergent’ above anything visible in the atoms that make them up;  cf. H2O, I rest my case), but rather of genes.   Okay, the analogy is loose, but no worse than that of genes & memes.  Indeed, motifs were the forerunners of the meme idea, and already much better thought out.  There is even a sort of folkloristic analogue of the allele:  the oikotype.
 
~


There is no reason to boggle at water as a single though scattered object,  the aqueous part of the world.  Even the tightest object, short of an elementary particle, has scattered substructure  when the physical facts are in.
-- W.V.O. Quine, Word and Object (1960), p. 98

In support of this:

(1) “the aqueous part of the world”:  cf. “empty space”, an anything but simply-connected entity (object).
(2) “scattered substructure”:   Unsure quite what he meant by this -- quarks are substructure of hadrons, but were unknown -- nay, unhypothesized -- in 1960, the publication-date of Quine’s classic.   However, a “smeared-out” (not really ‘substructural’)  nature of something so tiny-tight as the electron (still regarded as truly elementary) was suggested already


~     ~     ~     ~     ~

Postscript:   These are the posts so far that have touched on ontology. These largely concern mathematical Platonism, which we won’t focus on here,  other than to say that Quine’s quip ("To be is to be the value of a variable"), suitable for recital to the babe in the cradle, is trickier than it sounds, owing to his notion of substitutional quantification, which does not express existence, vs. objectual quantification, which does.


[Footnote] Contra-Quine:

It is not true that ideas face the bar of reality as corporate bodies:  rather, in the past, they evaded reality  as corporate bodies.    This word has, so to speak, a turnover ontology.  The “objects” (ie. the terms in which we classify the continuum of experience  into “things”), are not there for keeps.  In trying to handle … the continuum of experience, it is … proper to experiment with … diverse ways of clustering the flux into “objects”.
-- Ernest Gellner, Plough, Sword, and Book (1998), p. 64f.

Tuesday, March 4, 2014

An update re Mathematical (and Natural) Definition (re-updated)


[Continuing this essay:


While we’re on the subject, let us consider further the question of definition in mathematics.


Re Hilbert’s approach to the axiomatization of geometry:

Rather than defining points or lines at the outset  and then postulating axioms that are assumed to be valid for them, a point and a line were not directly defined, except as entities that satisfy the axioms postulated by the system.
-- Leo Corry , “The Development of the Idea of Proof”, in Timothy Gowers, ed., The Princeton Companion to Mathematics (2008), p. 139

This is not quite so radical or ‘post-modernist’ as it might sound, since traditional grammar recognizes many analogous cases in natural language, under the rubrics of synsemanticsyncategorematic and implicit definition.  [See posts with the Label "incomplete symbol".]  It is a relative notion, with a sliding scale;  but analysis will suggest that a very large set of words and multiword expressions (as, the use of a word in an idiom, especially in an opaque idiom) partake of some degree of syncategorematicity.   However, in the particular perspective of mathematics, this idea harmonizes especially well with a logicist or formalist approach to the subject:

The use of undefined concepts  and the concomitant conception of axioms as implicit definitions  gave enormous impetus to the view of geometry as a purely logical system.
-- Leo Corry , “The Development of the Idea of Proof”, in Timothy Gowers, ed., The Princeton Companion to Mathematics (2008), p. 139

Again, this is much less disorienting and self-bootstrapping than it may seem, since -- outside, indeed, of formal contexts -- virtually all of natural language works exactly like that; and not only expressions like whereas, the moreso as, French ne, German doch, which wear their syncategorematic character on their (empty) sleeves, either:  but plain words like bunny.   You do not learn to use such words on the basis of a definition, formal or informal -- however much it might please linguistic philosophers to invent a terminus technicus such as “ostensive definition”, which labels a phenomenon (or passle of phenomena) without explaining it.   For, as we have seen in our discussions and parables related to matters Quinean, these don’t really work, not logically;  they work pragmatically, to the extent that they work at all, because (since we are all molded from the same clay; or  if you prefer, since our bloodlines have all been subjected to the rigors of Natural Selection) we are all cut to the same cloth.  (To the extent that some individuals fall outside the innate cognitive norms, they fail to acquire the same semantics that the rest of us do:  or else, like some gifted and industrious autists, they acquire this only by dint of an artificial study, like someone learning Sumerian logographics.)   Thus, the following Onomastic Primal Scene does not actually obtain in any real nursery:

That, Timmy” (pointing -- but at or towards what?) “is a rabbit (noun count, singular).   And by this -- attend now, and please do not misunderstand me -- I do not intend to indicate the entire scene embracing carrots and furballs and playpen and binky (who left that there?) etc., let alone the cosmos as a whole (after all, one has to point somewhere), whether by itself or considered as but one flaky layer in the whole baclava-like complexus known as the multiverse;  but only the, er, furball-related entity.   And by this, I do not mean, so much, (although I do not literally not mean it, either), a pointlike or infinitessimal space-time slice of a leporiform trajectory along the world-sheet, nor a “thickened” (perceptually available) neighborhood of the same;  nor a sort of puddle of rabbit-stuff, undifferentiated from the rest of the puddle; nor a concrete instantiation of the Platonic Form, ‘Rabbit’;  nor a subobject in the Category Leporidae;  nor an agnostically structured pointset consisting of Undetached Rabbit Parts (although I sort of mean that, since, at some point, once you have detached the poor critter to bits  and scattered its disjecta membra over the face of the earth, to be eaten by vermin and recycled as independent atoms, -- at some point, we can no longer confidently say, “That is a rabbit”, in the sense of noun count, singular),  nor -- well, dash it all, I mean just Fluffy, okay?  Fluffy and other creatures that look and hop and act like her.  And by the way it looks like Fluffy wants a cuddle or something, because she is spritzing the wood-shavings in a semantophobic panic.”

~

The scenario above  comports more naturally with a coherence theory of truth, rather than a correspondence theory.

~

The quirky, philosophically-minded Intuitionist mathematician Brouwer, harbored similar “mysterian” views on ultimate indefinabilty:

In Brouwer’s opinion, mathematical definitions should not be looked upon in a mathematical way, but should only be used as a support for our memory.  Basic concepts, such as ‘continuous’, ‘once again’, ‘etcetera’, have to be irreducible.
-- Dennis Hesseling, Gnomes in the Fog:  The Reception of Brower’s Intuitionism in the 1920s (2003), p.  45

Nooit nog heeft door de taal iemand zijn ziel aan een ander meegedeeld;  alleen ein verstandhouding, di toch reeds is, kan door de taal worden begleid.

(Caption quotation:  op. cit., p. 32.)
~

Since, outside of the classroom, new words are almost never introduced explicitly, let alone lexicographically or metalinguistically, we must conclude that the language-learner somehow gets the right idea “from context”.   This notion is more problematic than might appear.

For, the history of contexts met-with over the course of a learningful life,  varies considerably from person to person (my own nursery school was wonderfully bunny-rich -- unless the creatures were actually guinea-pigs, come to think of it:  I no longer recall, and after all had nothing to compare them with at the time, they were simply our class mascots and Furry Friends -- but sadly penguin-deficient (of that I am quite sure);  nay, my lifelong platypus-deprivation has been nothing short of absolute), and the fact that we can happily chatter away  among our fellows  about all creatures great and small, without needing to resort to pointing at picture-books (although I do always carry a bunny-book about with me, just in case I should run into Wittgenstein) or red-faced arm-flapping exasperation as we attempt just one more time to make ourselves understood to our perversely thick-witted interlocutors (“Not a ‘triplex of mutually orthogonal rabbit-slices’, dammit!  I mean three  separate  rabbits !!”) strongly suggests that we come from Nature’s Nursery with a lot of shared ontology inborn.  (Chomsky’s school reached similar conclusions many years ago, by a somewhat different path.)

Two-dimensional representation of an imaginary rabbit.  Question:  What is the dimensionality of the *actual* imaginary rabbit?

~

Back to mathematics.
Here definition, in contemporary use, is the intuitive idea, to which axiomatization is the formal counterpart.    You define the term group by simply listing the axioms which any set endowed with an operation must satisfy  if it is to aspire to that dignity.
Yet, having made this move, we see that a vagueness was lurking in our original intuition:  since being ‘axiomatizable’ comes in various flavors:  finitely axiomatizable, axiomatizible in first-order bzw. second-order logic, etc.   And we find surprises, such as when so familiar an item as a torsion group  turns out not to be finitely axiomatizable within first-order logic.   Yet we know what we mean by it, for all that.


Compare: 
mammal:  definable in (cladistic) terms of shared descent
reptile: not so definable

water: definable in terms of molecular composition
blood, wine : not so definable

quartz : definable in terms of mineral composition
granite :  only approximately so definable, or definable at one remove.


~

How you define a mathematical item -- we may even say, how you go about defining it, the tack you take in trying to define it -- depends upon what you are intuitively aiming at.

For example:  How to extend the definition of the multiplication of a finite set of factors, to the infinite case?  (We did so for the case of convergent infinite sums without difficulty.)

Because of the special properties of zero with respect to multiplication, the most obvious definition of a convergent infinite product is not the valuable one.
-- Andrew Gleason,  Fundamentals of Abstract Analysis (1966)

Or again, from the great Gleason,  ever alert to the lexicographic aspects of mathematics:

The Bolzano-Weierstrass property is often taken as the defining property for compactness, since it is frequently the handiest property  for dealing with compact metric spaces.  However, it is not equivalent to the Heine-Borel property in general topological spaces, and it turns out that the latter is the more valuable in the general case.
-- Andrew Gleason,  Fundamentals of Abstract Analysis (1966), p. 269

Coming up with a useful definition (and here the “coming up with” does seem closer to invention than discovery) becomes an interesting question in its own right, and not a matter of mere fiat. 

Again compare:
Finding a definition (or, really, “characterization”;  yet ultimately the ink-stained lexicographer must needs still define) of:  Romanticism, Minimalism, Idealism;  joke, game; mollusc, microbe, plant;  silver, beige;  etc.


~

So for instance, let’s take logicism.

There is evidence that, in 1899, Hilbert endorsed the viewpoint that came to be known as logicism.  Logicism was the thesis that the basic concepts of mathematics are definable by means of logical notions, and that the key principles of mathematics are deducible from logical principles alone.
-- José Ferreirós, “The Crisis in the Foundations of Mathematics”, in Timothy Gowers, ed., The Princeton Companion to Mathematics (2008), p. 143

So there you are:  A nice clean definition.  Think what you will of the thesis, and come what may by way of later evidence pro or con, the definition is what it is, right?
Wrong.
Our author goes on:

Over time, this thesis has become unclear, based as it seems to be on a fuzzy and immature conception of the scope of logical theory.  … Historically speaking, logicism was a neat intellectual reaction to the rise of … the set-theoretic approach.

So!  In addition to being confirmed or refuted, apparently a thesis can decay, lose its sharp edges, like an unrefrigerated vegetable.   For:  Any definition of X  itself takes for granted the well-definedness of certain understood entities Y, Z …  Should the latter fall foul of better understanding, X itself can be left high and dry.

The consider the following definitions:

phlogiston:  a material which is the source of light and heat attendant upon combustion
phlogisticated air:  air mixed with phlogiston
monokeratic phlogisticene :  phlogiston mixed with powdered unicorn hoof  (cures scrofula and gout)

These delightful definienda, whose delineation was once so clear, have each met with a sad fate.
Definitions, like dephlogisticated unicorn-hoof, are liable to crumble into dust with the passage of time.

Thus, in mathematics:  Newton’s fluxions, etc.

~


Example of a definition  introduced in full awareness that it is merely provisional:

This definition of an affine algebraic variety should be considered only a working preliminary definition.  The problem is that it depends on considerations extrinsic to the objects themselves, namely the embedding of the affine variety in the particular affine space Cn.
-- Karen Smith et al., An Invitation to Algebraic Geometry (1998/2010), p.

This definitio (taking this in the actio rather than the actum sense) is in the spirit of Lakotos’  Proofs and Refutations.

~

Mathematics often sharpens our understanding of any pre-existing conception (“continuity”, “dual”) that comes to swim within its ken.  And so it is for the very notion of definition :  long assumed a matter of free choice, until Russell’s Paradox brought matters up short.   Whereupon he and Poincaré worked out their understanding of impredicative definition or impredicativity. 
Thus, in one formulation of Poincaré’s predicativist  approach:  “All mathematical objects (beyond the natural numbers)” (these being, as even Kronecker concedes, God-given) “must be introduced by explicit definitions.”  And, not just any definition you take a fancy to will do: 

If a definition refers to a presumed totality  of which the object being defined is itself a member, we are involved in a circle:  the object itself is then a constituent of its own definition.
-- José Ferreros, “The Crisis in the Foundations of Mathematics”, in Timothy Gowers, ed., The Princeton Companion to Mathematics (2008), p. 146

And this, you understand, is Very, Very Bad.  (We might cheekily dub it Definitional Incest.)


~

Mathematicians, like philosophers, and unlike anyone else (including even lexicographers), are given to a certain semantic Akribie --  an extraordinary self-critical attention to their own use of language.   As, consider this:

The conservation “laws” of momentum and angular momentum  are also readily introduced …
-- Robert Hermann, Differential Geometry and the Calculus of Variations (1968), p. 100

I have no idea what subtle mental reserve caused to author to quarantine the word laws in sneer-quotes, nor why he felt it necessary so to caveat -- so to signpost the approach to a possible Occasion of Semantical Sin -- in a work aimed (according to the preface), not at philosophers, nor Jesuit spiritual directors, nor even mathematicians, but to engineers and physicists (those are the grease-stained guys tinkering under the accelerator).   But the fact is, if you move in mathematical circles, your semiotic conscience becomes exquisitely sensitive and attuned.

~

In focusing on definition, I am inadvertently revealing the déformation professionelle of one who used to earn his bread (or rather his hardtack; the profession is ill-paid) as a lexicographer.   For, rather than trying to say what a thing “is” (and here the Korzybskian strictures against the copula  have their full force), we may say, pragmatically rather than ontologically, what a thing is for.   Thus, a hammer “is” a manufactured object of a certain range of shapes and weight, classically with a metal head and wooden handle, (etc. etc. -- “Etc.”, as the Korzybskians have it), if that is helpful to you;  but it is for driving in nails.

Thus -- to take a couple of concepts that always somehow puzzled me definitionally :

Chains and partitions of unity  free our proofs  from the necessity of chopping manifolds into small pieces.
-- Michael Spivak, Calculus on Manifolds

Now that is something a kitchen-maid could understand.

~

[Weiteres zum Thema]

On provisional/dialectical definition:

Menger wrote, in a series of papers on foundational questions  published in 1928:

Dabei möchte ich betonen, daß ich das Wort ‘Konstruktivität’ für ein  wenn überhaupt, so  vermutlich  auf verschiedene Arten und in verschiedenen Abstufungen  präzisierbares (bisher noch nicht präzisiertes)  Wort halte.
-- quoted in Dennis Hesseling, Gnomes in the Fog:  The Reception of Brower’s Intuitionism in the 1920s (2003), p. 199

(For logophiles only:  Let us here salute and savor  that phrase,  “ein  wenn überhaupt, so …”   Impossible to translate this into English  in so compact a compass.)

~


Dennis Hesseling, Gnomes in the Fog:  The Reception of Brower’s Intuitionism in the 1920s (2003), p. 14, quotes Lebesgue:

Bien que je doute fort  qu’on nomme jamais un ensemble qui ne soit  ni fini, ni infini,  l’impossibilité d’un tel ensemble  ne me paraît pas démontré.

Quite aside from the mathematical content to this, as sheer semantic content  that will baffle anyone who
(a) has learned the terms finite and infinite as simple contradictories (infinite iff not-finite);  and who
(b) accepts the tertium non datur
it seems a mere tautology, like the analytical-philosophical lore  of bachelors and married-men.
But this is from Lebesgue, note, as familiar with the intricacies of the various infinities  as anyone on earth.  Clearly something subtler here is meant.  Something I’d never heard of before -- the first worry of the Continuum Hypothesis, so I had understood, concerned the possible existence of wiggle-room between countable infinite and the cardinality of the continuum.


Quite possibly, however, since Lebesgue and Brouwer sometimes shared an intellectual orbit, the explanation may be sought in the following hint (op. cit., p. 66):  “Brouwer distinguishes between species which are abzählbar, zählbar, auszählbar, durchzählbar, and aufzählbar, where some of the distinctions  are related to the question of decidability.”


~

Another parallel between mathematics and (e.g.) biology, as regards a certain type of ‘definition’.
Sometimes you are not trying to focus on a new concept in splendid independence, giving necessary and sufficient conditions to ‘be an X’, de-fining (demarcating) its boundaries (Jordan-curve-fashion) between what-all is inside  and what-else is out;  but, rather, starting from some homely, antecedently-familiar item Y, to define this new X as being similar to that Y.    Sometimes you say they’re similar, and leave it at that:

            A hare is like a rabbit.
            A coot is kind of like a duck.

Sometimes you add differentia:

            A zebra is like a horse with stripes.

Or, you may say that the new concept X generalizes Y, without giving necessary or sufficient conditions for membership in the generalization, with or without further examples of members of X:

            Amphibians form a taxon of animals that includes frogs.  (They ‘generalize’ the frog.)
            Amphibians form a taxon of animals that includes frogs and salamanders.

All these strategies are (so to speak) topologically distinct, the one from the other.

Compare, in math (an actual textbook example):

Locally convex spaces are topological vector spaces that generalize normed spaces.

Here the relatively exotic new concept “locally convex spaces” plays the role of amphibians in the example above, with the normed spaces (familiar from the nursery) filling that of our friends the frogs;  with an additional delimiter, topological vector spaces, basically saying:  “generalize, but not too far”.   Thus, if we said

Vertebrates form a taxon of animals that includes frogs.

that would still be a true statement, but the belt would have been let out too many notches to hold up the conceptual trousers.