Skepticism about induction happens only among academic philosophers, and only in print. Tell an induction skeptic to take a long walk off a short dock or hint that his health insurance will be cancelled and you will find an immediate and angry convert to Realism.
Some philosophers come to their skepticism about induction from puzzles which they are unable to solve and reason that, since they cannot solve the puzzles, it’s a good bet to side with skepticism. Well, in some ways this is natural.
A classic puzzle is Nelson Goodman’s “grue”. Goes like this. Grue is a predicate, like green or blue, but with a built-in ad hoc time component. Objects are grue if they are green and observed before 21 October 1978 or blue and observed after that date. A green grape observed 20 October 1978 and a blue bonnet observed 22 October 1978 are grue. But if you saw the green grape yesterday, or remember the blue bonnet from 1976, then neither are grue. The definition changes with the arbitrary date.
So imagine it’s before the Date and you’ve seen or heard of only green emeralds. Induction says future, or rather all unobserved, emeralds will also be green. But since it’s before the Date, these emeralds are also grue, thus induction also says all unobserved emeralds will be grue. Finally comes yesterday—and lo!—a green and not a blue emerald appears, thus not a grue emerald. Induction, which told us it should be grue, is broken!
There have been several exposures of the grue fallacy before, and up until the other day (another date!) I had thought David Stove’s in his Rationality of Induction was best. But I now cast my vote for Louis Groarke’s in his An Aristotelian Account of Induction. He calls belief in Goodman’s fallacy “an adamant will to doubt rather than an evidence-based example of a deep problem with induction” and likens it to the fallacy of the false question (e.g. “Have you stopped cheating on your taxes yet?”).
Groarke says (p. 65):
The proposition, “emeralds are grue,” [if true] can be unpacked into three separate claims: emeralds are green before time t (proposition1); emeralds are blue after time t (proposition2); and emeralds turn from green to blue at time t (proposition3). Goodman illegitimately translates support for proposition1 into support for proposition2 and proposition3. But the fact that we have evidence in support of proposition1 does not give us any evidence in support of all three propositions taken together.
What does the arbitrary time have to do with the essential composition of an emerald? Not much; or rather, nothing. The reason we expect (via induction) unobserved emeralds to be green is we expect that whatever is causing emeralds to be green will remain the same. That is, the essence of what it is to be an emerald is unchanging, and that is what induction is: the understanding of this essence, and awareness of cause.
Groarke emphasizes that the time we observe something is not a fact about the object, but a fact about us. And what is part of us is not part of the object. Plus, the only evidence anybody has, at this point in time, is that all observed emeralds have been green. We even have a chemical explanation for why this is so, which paradox enthusiasts must ignore. Thus “there is absolutely no evidence that any emeralds are blue if observed after time t.”
Two things Groake doesn’t mention. First is that, in real life, the arbitrary time t is ever receding into the future. I picked an obviously absurd date above; it’s absurd because we have all seen green emeralds but no blue ones up to today, which is well past 1978. The ad hoc date highlights the manufactured quality of the so-called paradox. When, exactly, should we use a grue-like predicate for anything?
Secondly, nobody not in search of reasons to be skeptical would have ever thought to apply a predicate like grue to anything. It is entirely artificial. If you doubt that, consider that you can substitute any other predicate after the arbitrary date. It doesn’t have to be blue. Try salty, hot, tall, or fast. An emerald that is green up until t then fast? That’s ridiculous! Yes, it is.
After showing the paradox isn’t, Groake goes on to explain the possible reasons why the paradox has been so eagerly embraced. Cartesian corrosion. That bottomless skepticism which dear old Descartes introduced in the hope of finding a bedrock of certainty. There isn’t space here to prove that, but anybody who has read deeply in epistemology will understand what that means.
Update A glimpse of how much angst the “problem” of grue has created, try this (or similar) searches. Also note the New & Improved title.