Prima facie some people might think that Aquinas’s epistemology is susceptible to the brain in a vat (BIV) objection. Two things could suggest this: (1) Aquinas is a direct realist (in something like John Searle’s sense of the term minus the biologism and perhaps a few other things) and (2) he believes that, for human beings, it is our senses that first put us into cognitive contact with the world (and in which all our subsequent natural knowledge of things is rooted).
But I don’t think that Aquinas really needs to worry about BIVs. He doesn’t need to because nobody needs to, that is, no reasonable person does. But before I tell you why I think this, let’s consider the objection.
If I may, I shall use Hilary Putnam’s formulation of the objection as he presents it in Reason, Truth, and History. Putnam, of course, thinks he has an argument that shows that the BIV objection is self-refuting. I’m not going to adopt Putnam’s argument or evaluate it. I’m just going to use his account of the objection. Here it is:
Imagine that a human being (you can imagine this to be yourself) has been subjected to an operation by an evil scientist. The person's brain (your brain) has been removed from the body and placed in a vat of nutrients which keeps the brain alive. The nerve endings have been connected to a super-scientific computer which causes the person whose brain it is to have the illusion that everything is perfectly normal. There seem to be people, objects, the sky, etc.; but really, all the person (you) is experiencing is the result of electronic impulses travelling from the computer to the nerve endings. The computer is so clever that if the person tries to raise his hand, the feedback from the computer will cause him to “see” and “feel” the hand being raised. Moreover, by varying the program, the evil scientist can cause the victim to “experience” (or hallucinate) any situation or environment the evil scientist wishes. He can also obliterate the memory of the brain operation, so that the victim will seem to himself to have always been in this environment. It can even seem to the victim that he is sitting and reading these very words about the amusing but quite absurd supposition that there is an evil scientist who removes people’s brains from their bodies and places them in a vat of nutrients which keep the brains alive.
Against direct realists like Aquinas, who hold that, in general, our senses reliably deliver an extra mentem world to us, the objection is supposed to show that they can never be certain that this is the case, they can never be certain that such a knowable extra mentem world exists. Cue the dark chuckling of the skeptic.
But why are Aquinas and other direct realists condemned to this uncertainty? I take it that it is essential to the BIV objection that, by hypothesis, there would be no noticeable difference between the worlds experienced by us whether the ontological/epistemic situation is as the direct realist thinks or whether we are BIVs. In other words, in both worlds, things would look exactly the same. This being so, the poor direct realists just can’t say for sure that they aren’t BIVs. And, presumably, they should give up their direct realism or at the very least not be too smug about it.
I’m afraid, however, that I’m just not convinced that direct realists should be all that anxious in the face of the BIV objection. The reason why I think this way is easy to state. According to the hypothesis, you could never have any evidence that you are a BIV, for if you did, you could know you were a BIV. You might understand that it is a logical possibility, but it is a logical possibility for which you have zero evidence and (again, according to the hypothesis) never could have any evidence. In the Enquiry Concerning Human Understanding, David Hume tells us that “[a] wise man ... proportions his belief to the evidence.” Who could disagree? But if I should proportion my belief to the evidence, then I should never take seriously the proposal that I am a BIV. And if I am a direct realist, I should never give up my direct realism because of it.
Consider an analogous case. The controversy over Shakespeare’s real identity is well-known. It seems to me to be logically possible that the real Shakespeare is a Norwegian playwright who became proficient in English. But suppose no evidence ever turns up that suggests that this is more than a logical possibility. Without any evidence, the Shakespeare scholar who takes the Norwegian possibility seriously is a perfect fool.
Aquinas and other direct realists need not worry about BIVs (nor need anybody else).