Monday, November 29, 2010

Rough Jottings on Zeno's Argument and Paradoxes Generally

Graham Priest has an article on dialetheism and paraconsistency at the NYT's "Stone" blog. I was struck by what he says about Zeno's paradox:

Here we can’t just accept the conclusion: we know that the car can get to point B. So something must be wrong with the argument. In fact, there is now a general consensus about what is wrong with it (based on other developments in 19th-century mathematics concerning infinite series). You can do an infinite number of things in a finite time — at least provided that these things can be done faster and faster.

The consensus, which is sometimes called the Standard Solution, can only get "You can do an infinite number of things in a finite time - at least provided that these things can be done faster and faster," if it provides a solution to the paradox, not vice versa. And it's not so clear that the Standard Solution does everything that would be required to resolve/solve/dissolve the Paradox. And, indeed, when you look at elaborations of how the Standard Solution is supposed to solve the paradoxes, such as the IEP article on the subject, all the responses very clearly end up being, "Wherever there's a problem, we assume that you can do what Zeno denies can be done, and make a distinction based on that assumption." Which is fine, but one should be quite clear that this is not what solving a paradox is: to make this response work you need not only to show that you avoid the paradox and have provided a consistent answer; you need also to show that there is independent good reason to reject the assumptions of the paradox you are rejecting. People have given the conclusion Priest gives for literally ages -- Aristotle explicitly mentions it in response to the Dichotomy. What has always been in question is what can support this conclusion without begging the question. This is why Aristotle makes his famous distinction between actual and potential infinites here: actuality and potentiality are more fundamental concepts than anything in the Paradox itself, and are presupposed by the Paradox itself, and so can be used simultaneously to argue that its assumptions are wrong and that an alternative is right. Whether one agrees with Aristotle as to the actual answer or not, this is the way to dissolve a paradox. The Standard Solution doesn't do this, and the usefulness of the mathematics that yield it is neither here nor there: it always remains open to accept both the Standard Solution and the Paradox by being an anti-realist about the former and a realist about the latter. This is a regular problem with mathematical solutions to non-mathematical problems: we know for a fact that extremely useful mathematical solutions can fail to correspond to reality, and therefore it's always an option to be anti-realist about any of them. The Standard Solution at most shows you that, if certain things are true about change, then a consistent system can be had in which Zeno's Paradox can't be formulated. It doesn't show that those things are true about change, but at most that if we at least pretend that they are we get right answers without having to worry about the Paradox. The Paradox is evaded for practical purposes, which is good, and the evasion is shown not to be inconsistent with itself, which is very good, but if you leave it at that, the Paradox has not been resolved. Indeed, if you leave it at that, you clearly show that you have no idea what resolving/solving/dissolving a paradox is. And showing that every feature of the (extraordinarily complex) Standard Solution has real, independently establishable, physical counterparts is a massive challenge that no one has ever undertaken.

It reminds me a little, actually, of common responses to the Preface Paradox, which was talked about at length in the blogosphere some time back. The usual response was to posit as true whatever was required to make there no longer be a paradox. This is an evasion of the paradox, and a perfectly reasonable thing to do for practical purposes, but it doesn't deal with the paradox at all: what you need to do is prove that what you are positing really is true and doesn't just save the appearances. And when you set out to do that you find that independent proof of the assumptions they make is extraordinarily difficult (and that many of the proposals for handling the paradox that are taken by their proponents as just obvious are inconsistent with each other).

It's also not really correct to say that the Standard Solution is a general consensus: there is no general consensus. The Standard Solution is easily the dominant single position, but it has quite a few robust rivals, and when you take those into account it's not even always clear that the Standard Solution is accepted by a majority of people. Lots of people still prefer Aristotelian approaches of various kinds, at most thinking that they need to be refined; lots of people are constructivists; lots of people prefer appeal to infinitesimals; and so forth. No one of these groups is even close to being as dominant as the Standard Solution group, but all together make a very sizable bunch. What we have is a dominant proposal; this is very different from a general consensus.

Further, Zeno's Paradox wasn't an argument that there could be no consistent mathematical system without the Paradox; it was an argument that actual motion through actual space and time was something opponents of Parmenides and Zeno (it is not, and never has been, clear whether Zeno was attempting to defend Parmenides or merely to criticize the arguments proposed against Parmenides, because we don't know how closely Zeno himself actually followed Parmenides) could not coherently account for. This is as certainly known as anything else about Zeno's Paradox; it's one of the few things that is agreed on by the three major interpretations of the Paradox (that Zeno was defending Parmenidean monism against common-sense pluralism; that Zeno was a nihilist attacking both monism and pluralism; that Zeno was not defending any particular position but only raising problems). The genuinely important question is not, "Is there some set of assumptions that, if true, would dissolve the paradox?" because the answer to that is "Of course," regardless of the paradox. The question, "What proves these dissolving assumptions actually true?" is more important, but is still not the most important question. The most important question is: "What are the common features of the accounts of motion Zeno's Paradox on its own makes it impossible to accept?" That is, people often look at paradoxes upside down: they focus on finding things that at least evade the paradox. There are always a great many of those, and sometimes the answer that dissolves the paradox is actually not interesting, beyond dissolving the paradox. What is really and consistently valuable in a paradox -- and it always remains even when the paradox is dissolved -- is what it tells us about what we can't accept.