Wednesday, July 16, 2008

Parsons's Mathematical Thought: Sec. 12, Nominalism

This is a short and rather insubstantial section, which I'm just taking separately to get out of the way, because the next section is weighty (and one of the longest in the book).

Parsons understands 'nominalism' Harvard-style -- no surprise there, then! -- to mean the rejection of abstract entities and the eschewing of (ineliminable) modality. What hope, then, for giving a response to the potential-vacuity problem for eliminative structuralism about arithmetic (say) which meets nominalist constraints? We can't, by hypothesis, go modal: so what to do?

Well, as the physical world actually is (or so we might well now believe), there are in fact enough physical things -- e.g. space time points -- and suitable physical orderings on them to give us physically realized 'simply infinite' structures. But Parsons is unhappy with this way of meeting the vacuity worry, and for familiar reasons: "[S]hould it be taken as a presupposition of elementary mathematics that the real world instantiates a mathematical conception of the infinite? This would have the consequence that mathematics is hostage to the future possible development of physics."

But (although I have no particular nominalist sympathies myself), I'm not sure how worried the nominalist eliminative structuralist should be about giving such hostages to fortune. As things are, given how we believe the world actually to be, he can reasonably continue to speak with the vulgar and treat arithmetical claims as true or false. Even if the worst happens, so we come to believe the world is ultimately grainy and finite in all respects, it's not that 'school-room' arithmetic is going to get undermined. At most, it is the idealizing rounding out of school-room arithmetic which insists on an infinitude of numbers. And if it should emerge that the rounding out, construed the eliminative-structuralist way, collapses in vacuity -- well, formal arithmetic can still be played as an intriguingly entertaining game. It's just that then, after all, the nominalist eliminative structuralist who is relying on physical realizations for structures can no longer readily construe idealized arithmetic's claims as true or false, and so the nominalist has to sound a bit more revisionary. But, he'll say, so what? (Parsons says "a great deal of the historically given mathematics would have to be jettisoned in this case" -- but that's too quick. Talk of 'jettisoning' covers over a slide. For no longer thinking of arithmetic as construable as literally true by the eliminative structuralist manoeuvre is not the same as throwing arithmetic into the trash-can, as any fictionalist will insist.)

What about the other line that offered to the nominalist at the end of Sec. 11? -- i.e. sidestep the vacuity problem by going modal in an anodyne way ("interpret the theories in an if-thenist way, but deal with the problem of possibility by appealing to consistency, nominalistically interpreted"). Well, again Parsons sees trouble, this time arising from the fact that there might be physical limitations in how big a proof-token could be, and so a theory could count as (nominalistically) consistent -- because no proof of an inconsistency could be tokened -- even if we can show that there is a process which, given world enough and time, would produce an inconsistency. But again, I'm not sure that the obstreperous nominalist couldn't swallow that too.

At the end of this section, Parsons revisits the question of how to frame an eliminative structuralism for arithmetic. He looked at a move from a set-theoretic formulation to a more 'logical', second-order formulation. But could we go first-order, in a way more congenial no doubt to those of nominalist inclinations? The trouble is, of course, that we won't get categoricity (whatever we build into the axioms), so the eliminative structuralist who goes first-order runs up against the intuition that the natural numbers have a unique structure. But how secure, in fact, is that intuition? Parsons raises that excellent question (too often passed over in silence), but only to shelve it until Ch. 8. So we'll have to return to that later.

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