If anyone knows enough epistemology or philosophy of science, I'm wondering if there's an orthodox thing to say about explanations in cases like the following:
You're a hippie, lying on your back in a meadow where birds are fairly uncommon. You're carrying a drug that will cause you to have a visual hallucination of a flock of birds flying above you, but leave your reasoning abilities as normal. You take the drug, and soon enough, you have a visual experience as of a flock of birds flying above you. If you had experienced this without taking the drug, you would've believed that lots of birds really were flying overhead. But you know that you did take the drug.
I think it's pretty clear that in this case, it's rational to believe that there are no birds up there. Even though a causal story that implies the existence of birds would explain all the observational data, and even though you have no counterevidence against the birds' existence, you have no reason to accept the existence of the birds. All your observational data is already explained by something you have good reason to believe -- namely, that the drug has given you illusory bird sensations. If you have one good explanation of something, you don't keep buying other explanations and accepting them in conjunction with the first one. Maybe you accept an inclusive disjunction of them if other reasonable explanations are offered, but you don't jump to accepting the conjunction without further evidence.
So here's my question: What general principle of explanation do we appeal to in rejecting the real-birds explanation in the above case? One possibility is simplicity. You take on existential commitments in making explanations -- commitments to the existence of a causal mechanism by which the drug influenced your sensations, for instance. Occam's razor will slice away redundant existential commitments like a commitment to real birds.
I post this because I have a similar example in a paper, and I thought simplicity was doing the work, but Brian Leiter thought that was weird. If anyone has a better idea of what principles are involved here, I'd like to know!
You're a hippie, lying on your back in a meadow where birds are fairly uncommon. You're carrying a drug that will cause you to have a visual hallucination of a flock of birds flying above you, but leave your reasoning abilities as normal. You take the drug, and soon enough, you have a visual experience as of a flock of birds flying above you. If you had experienced this without taking the drug, you would've believed that lots of birds really were flying overhead. But you know that you did take the drug.
I think it's pretty clear that in this case, it's rational to believe that there are no birds up there. Even though a causal story that implies the existence of birds would explain all the observational data, and even though you have no counterevidence against the birds' existence, you have no reason to accept the existence of the birds. All your observational data is already explained by something you have good reason to believe -- namely, that the drug has given you illusory bird sensations. If you have one good explanation of something, you don't keep buying other explanations and accepting them in conjunction with the first one. Maybe you accept an inclusive disjunction of them if other reasonable explanations are offered, but you don't jump to accepting the conjunction without further evidence.
So here's my question: What general principle of explanation do we appeal to in rejecting the real-birds explanation in the above case? One possibility is simplicity. You take on existential commitments in making explanations -- commitments to the existence of a causal mechanism by which the drug influenced your sensations, for instance. Occam's razor will slice away redundant existential commitments like a commitment to real birds.
I post this because I have a similar example in a paper, and I thought simplicity was doing the work, but Brian Leiter thought that was weird. If anyone has a better idea of what principles are involved here, I'd like to know!
8 comments:
I would think it involves some sort of (Bayesian?) probability calculation. Given the low "base rate" probability of real birds, and the very high chance of a hallucination, it's rational to conclude that it was just a hallucination. (Isn't it?)
Hey, that sounds pretty good!
Of course the Bayes stuff is nice. That's why all us economists and mathematicians have probability based outlooks on the world.
I always had the impression Neil, that you were looking for absolute answers.
(And no, that you have a majority or plurality probablity to believe in one outcome, does not mean you should say that occupies the 1 in a [1,0] set.)
Thanks, everyone. After reading Justin's comment, I've realized that the initial post was ambiguous. Was I trying to explain why one rejects the real-bird explanation, or why one rejects the existence of real birds, not considered as part of an explanation? It's actually the latter claim that I was particularly interested in. Of course, that implies some interest in the former, since the former implies the latter.
I'm actually not sure how the story works in Bayesian terms. Most of my knowledge about Bayesianism comes from just having read this Stanford article, so count me as a confused beginner.
Certainly, the base rate of birds is pretty low. But P(Birds/Bird-sensations) is pretty high. Now, you might say that the relevant thing to consider is P(Birds/Bird-sensations-when-you're-on-drugs), which ought to be low. But what I'm looking for is an explanation of that. What's the reason why the conditional probability of birds when you're having specifically drug-induced sensations is so low? I'm assuming that a low P(Birds/Bird-sensations-when-you're-on-drugs) isn't one of our priors, and that we have to build it out of some other stuff. So what is this other stuff?
A low P(X/X-isn't-part-of-the-simplest-explanation-that-explains-all-the-data) would do the work.
Great! Thanks.
There is actually a real-life example analogous to your thought experiment, in my mind at least.
Back in the dark ages, it was widely accepted that rocks fell out of the sky. Of course, it was the work of the devil. But then that newfangled scientific thinking took hold, and all of a sudden, people who believed that rocks fall out of the skies were dismissed as ignorant and superstitious. Except of course they were right.
I think the main thing to take from this is that 'pretty darn sure' is not the same as 'absolutely certain'. 'Conclusion' implies certainty. Take a hundred hippies on drugs, and some of them are bound to be seeing real birds as well as phonies.
Yes, Angelica... as Justin points out, seeing the bird-images raises the probability of real birds by 2.2%.
Hi, just stumbled across your blog from Rox Populi, where you had the funniest caption of the bunch.
My thought is, it's only LIKE Bayesianism AFTER the drug has worn off. If you're considering your judgment while the drug is affecting you, then you may well believe it's real birds.
But since Bayesianism is about prediction more than it is about ex post reasoning, then, semantically, you might be better off calling the doubt you experience afterwards as Pierce's "abduction," Lawson's "retroduction" or more generally as "inference to the best explanation."
Oddly enough, it's just this sort of IMPROBABILITY that would seem to cause real trouble for a Humean inductivist. He would continue to say, "just because it was entirely improbable that I would ever see birds, I can't be sure birds will never appear. And I saw birds, therefore how do I know the birds WEREN'T in fact real?" So, oddly enough, we're at certain times best off when we harbor strong convictions and erase doubts that might lead us into this sort of Humean trap.
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