One of my quirkier philosophical views is that the most pressing question in metaphysics, and perhaps all of philosophy, is how to distinguish between disjunctive and non-disjunctive predicates in the special sciences. This might look like a relatively technical problem of no interest to anyone. But I suspect that the question is important to all sorts of issues, as well as being one of those unhappy problems that no one seems to even have a beginning of a solution to. One of the issues that it’s important to was raised by Brad DeLong yesterday. He was wondering why John Campbell might accept the following two claims.
- There is an important and unbridgeable gulf between our notions of physical causation and our notions of psychological causation.
- Martian physicists—intelligences vast, cool, and unsympathetic with no notions of human psychology or psychological causation—could not understand why, could not put their finger on physical variables and factors explaining why, the fifty or so of us assemble in the Seaborg Room Monday at lunch time during the spring semester.
I don’t know why Campbell accepts these claims. And I certainly don’t want to accept them. But I do know of one good reason to accept them, one that worries me no end some days. The short version involves the conjunction of the following two claims.
- Understanding a phenomenon involves being able to explain it in relatively broad, but non-disjunctive, terms.
- Just what terms are non-disjunctive might not be knowable to someone who only knows what the Martian physicists know, namely the microphysics of the universe.
The long version is below the fold. (This is cross-posted to CT, so I’ve filled in more of the background than I usually would here.)
The story starts with some relatively banal observations about explanation and understanding.  Imagine that I throw a stone at a window, it strikes the window with momentum m, and the window breaks. Now one way to explain the window’s breaking is to say that it was struck by a stone with momentum m. But while that might be in some sense a complete explanation of the breaking, there are other explanations that promote greater understanding. This explanation doesn’t make explicit the salient fact that the window’s shattering was not particularly dependent on the precise momentum of the projectile, or that the projectile was a stone. Someone who explains the breaking in terms of it being struck with a projectile of momentum between m1 and m2, where these are the rough limits for what is (a) sufficient to shatter the window and (b) plausible given that I threw the projectile, seems to have a better explanation of the shattering, and a greater understanding of why the window shattered.
It’s tempting to conclude at this stage that if one explanation explains an event in terms of some particular being F, and another explains it in terms of that particular being G, where being F entails being G but not vice versa, then the second explanation provides a deeper understanding of what happens. In short, broader explanations (explanations that are made true in more ways) are better. But that principle seems to have clear counterexamples. Imagine a third ‘explanation’ that says the window broke because it was either struck with a projectile of momentum between m1 and m2, or struck by a window-breaking spell. This explanation is even broader than the two previous, since the truth of the explanans is entailed by, but does not entail, the truth of the explanans in the previous cases. But it is, intuitively, a worse explanation than what came before. It certainly doesn’t provide a deeper understanding. Indeed, someone who offers this ‘explanation’ has very little understanding of why the window broke.
Still, it seems a qualified principle might work. Broader explanations are better as long as the terms they use are not disjunctive. The idea that some terms are disjunctive and others aren’t goes back at least to Goodman’s Fact, Fiction and Forecast. Goodman famously defined up a new term grue. Something is grue, I’ll say, iff it is green and observed or blue and unobserved. As Goodman noted, observing lots of emeralds and seeing they are all grue provides us with no reason to think the next emerald we see will be grue. This kind of simple induction doesn’t work when dealing with terms like ‘grue’. Various authors, most importantly David Lewis have argued that the distinction Goodman pointed towards, between disjunctive terms like ‘grue’ and non-disjunctive terms like ‘green’, has many implications for across philosophy. Following tradition, I’ll call the ‘grue’-like terms gruesome, and ‘green’-like terms natural. (And I’ll often suppress the fact that the difference between gruesomeness and naturalness is a matter of degree, as there are a spectrum of cases in the middle.)
So what we want for understanding, I think, are explanations that are as broad as possible but not involving gruesome terms. Or perhaps explanations that strike the best balance between breadth and gruesomeness. I’ve only ‘argued’ for that by a single case, but the principle seems pretty plausible to me. At least it’s plausible enough to be the first half of my reason for believing a position like Campbell’s.
Now the relatively difficult part. It’s easy enough when talking about toy physical explanations to say what are natural and gruesome terms. It is, to say the least, somewhat harder to do the same thing in special sciences. Is the term ‘seasonally adjusted fall in sales’ natural or gruesome? By the standards some philosophers use, it is pretty gruesome because it is explicitly defined relative to calendar dates. (After Goodman’s work it wasn’t too hard to find philosophers saying this was a tell-tale sign of the gruesome.) But there are very good explanations that make use of terms like this. For a simpler case, is ‘weekday’ a natural or gruesome term? Again, it looks pretty gruesome from the perspective of micro-physics. But it is hard to explain/predict/understand traffic patterns without talking about weekdays and weekends. Progress here is not going to be easy.
David Lewis proposed that how gruesome a term is could be a function of the shortest definition of that term in terms of fundamental physics. But that also looks to rule out most special science explanations out of court pre-emptively. We have no idea how to define, say, a rise in demand for widgets in terms of micro-physics, but I bet that any such definition will be immensely complicated. So complicated that if we took Lewis’s idea seriously, we’d say that explanations of anything in terms of rising demand for widgets would be impossibly gruesome. But such explanations can be perfectly good. So Lewis’s idea fails.
The problem is that no one seems to have a better idea. One constraint on an answer is that we can’t use the contents of mental states in the answer, because there is good reason to think that the naturalness of various terms is part of what makes our mental states have the content that they have. (Again, see Lewis for the arguments for this.) The best answer I know of that doesn’t appeal to beliefs, intentions etc of humans is Lewis’s answer in terms of definition length. And that’s a non-starter I think.
So no one really knows how to answer, or even to make much of a start on answering, the following two questions.
- What makes a term natural rather than gruesome?
- How could one know that various terms, from biology, psychology, anthropology, economics, etc, are natural rather than gruesome? (In particular, could one infer the naturalness of various terms from a micro-physical description of the world?)
The two questions might be related. If there is an answer to the first question that relates the naturalness of special sciences to facts about micro-physics in some relatively straightforward (or at least Turing computable) way, then one way to know what’s natural and what’s gruesome would be to find out all the micro-physical facts, and then do the relevant computation. (That is, the answer to the parenthetical question is yes.) But we don’t know whether such a metaphysical story is true.
What we do know is that we didn’t find out that terms such as ‘cell’, ‘belief’, ‘society’ or ‘demand’ are (relatively) natural by their relation to micro-physics. One somewhat troubling (to reductionists like me) prospect is that the only ways to find out the naturalness of these predicates is something like the way we found out that they are natural. (Not that we know what that way is either.) And if that’s right, it’s possible that the Martian physicists can’t tell the natural from the gruesome terms in biology, psychology, anthropology, economics, etc.
If that’s all correct, then I think there’s a pretty good sense in which the Martian physicists won’t understand human behaviour. (Or, for that matter, the behaviour of all sorts of complex systems. The particular kind of way that human brains are complex might not be doing very much work in this argument.) They might know all sorts of counterfactuals saying that if certain objects were different in certain ways, then certain events would not have come about. But knowledge of such counterfactuals only consistitutes understanding if the ways in question are relatively natural. Remember, someone who takes the best explanation of the window’s breaking to be that it was either struck by a projectile with momentum m or a spell doesn’t really understand why the window broke. And that’s the case even though had the window not been struck by a projectile with momentum m or a spell, it wouldn’t have broken.
I think there’s a case to be made that, for all we now know, the Martian physicists will be in an analogous position with respect to understanding/explaining our behaviour. And that’s why I think, or at least worry, that Campbell’s position might be true. And it’s one of several reasons I worry about us lacking any story about what makes special science terms natural.
1 Banal these may be, but I wouldn’t have learned of them if not for discussions with Michael Strevens. Many of the important points relevant to this post are discussed in this paper (PDF), especially in section 5. Michael has a book on explanation coming out some time in the near future which I very highly recommend.