Chapter 7 of Timothy Williamson’s “The Philosophy of Philosophy” is an extended argument against “psychologising” evidence in philosophy. Before we can evaluate those arguments, it would be useful to get clear on just what it is to psychologise evidence. In this post I’ll say a bit about what that amounts to, and in the next post look a bit more carefully at Williamson’s text to see just what position he is attributing to his opponent.

In some ways the debate Williamson is contributing to is among the oldest in modern philosophy. Consider the following two positions about perceptual evidence, each of which has found many partisans over the last few centuries.

• Perceptual Evidence is Psychological. My perceptual evidence consists in facts about the psychological states I am in when undergoing a perceptual experience. So, for instance, my perceptual evidence might include that I’m visually representing that there is a table in front of me.
• Perceptual Evidence is External. My perceptual evidence consists in facts that I perceive. So, for instance, my perceptual evidence might include that there indeed is a table in front of me.

The psychological theory has a number of advantages. It can explain how people having illusory perceptions can get the same kind of evidence (albeit of lower quality) as people having veridical experiences. It arguably staves off certain kinds of doubts about our evidence, at least to the extent that we have privileged access to our psychological states. It explains the fact (if it is a fact) that when we get evidence in favour of some proposition _p_ about the external world, we generally know what kind of evidence we have. It is unusual, that is, to get evidence that _p_, but not know whether that is visual evidence, or tactile evidence, or testimonial evidence, or whatever. If the evidence for _p_ just is the visual or tactile or testimonial experience, that is easily explained. And it offers the prospect of an easy theory of evidence possession; a point I’ll return to below.

But there’s one big cost of the psychological theory: it seems to promote scepticism. There is a long tradition, starting in the modern period with Descartes, of proponents of the psychological view wondering how to get from psychological evidence to knowledge of the external world. And there is another long tradition, culminating at the present with Williamson, of opponents of the psychological view using this worry as a reason to start with evidence in the external world, and avoid this sceptical doubt.

The debate here is not confined to perception. We can have a similar debate in testimony. Imagine I am told that _p_ by a trusted friend. I now have some evidence for _p_. What is it? One answer, similar in spirit to the psychological answer above, is that I’ve been told that _p_. Another answer, similar in spirit to the external answer, is _p_ itself. The latter answer might be favoured by a theorist of testimony who thinks that when I get testimony from a trustworthy source, I simply receive the warrant they have for believing _p_. (The two answers here aren’t quite equivalent to the positions known as reductionism and anti-reductionism in the epistemology of testimony. Someone might be an anti-reductionist and hold that the telling, rather than what’s told, is the evidence, by holding that we don’t need any extra grounds to infer, on the basis of that evidence, that _p_. I’ll say more about such inferential rules in later posts.)

Both answers here are possible, but it is much more plausible to take the evidence to be the telling rather than what’s told. So we can use that as a relatively clear example of what happens when we take evidence to be something that supports an external world proposition _p_, rather than _p_ itself. One consequence is that in reporting inferences, we can replace testimonial knowledge with knowledge that the testimony was made without making the inference worse. So imagine we know that if Celtic win today, they’re champions, and we’re told by a trusted friend that Celtic did indeed win. Then we might make either of the following inferences.

An inference from facts about football

1. If Celtic won, they are champions.
2. Celtic won.
3. So, Celtic are champions.

An inference from facts about testimony

1. If Celtic won, they are champions.
2. My friend said that Celtic won.
3. So, Celtic are champions.

The first is valid, while the second is not. But we are interested here in inferences, not implications, so that’s no disqualifying mark against the second inference. For the second has a virtue not shared by the first, namely that its premises are more secure. So it looks like the two inferences are equally good. And that suggests that the second inference really is just making explicit the inference that’s underlying the first.

We’ve now said enough to set up the interesting debate about philosophical evidence. Often we say things like _Intuitively, that’s a cause of that_, or _Intuitively, that’s wrong_ and so on in philosophy. What kind of evidence are we appealing to here?

• Psychological States. Our evidence is that we have the intuition that such and such is a cause, or is wrong, or son on. In other words, our evidence is a certain psychological state.
• External Facts. Our evidence is not the intuition itself, but its content. So our evidence is that such and such _is_ a cause, or _is_ wrong, or whatever. When we say that this is intuitive, we are perhaps reporting how we have that evidence, we are not describing the evidence we have.

The philosopher who psychologises philosophical evidence is the one who says that our evidence is the first kind, the psychological states. So far it seems clear that this is the kind of view that Williamson has in mind. But we might wonder how far this goes. An extreme version of the view that philosophical evidence is psychological is that we could _always_ replace _p_ in a philosophical argument with intuitively _p_, without loss of argumentative force. Remember in the testimony case we replaced a fact we were told about with the fact of the telling and didn’t seem to make our argument any worse; the same goes on here.

That position is implausible in the extreme. It doesn’t need any fancy argument about the nature of evidence to see that it’s wrong, it simply mistakes the nature and scope of philosophy. To see this, consider an argument loosely based on Peter Singer’s _Animal Liberation_.

1. Eating meat is morally permissible only if it is necessary for a healthy diet
2. Eating meat is not necessary for a healty diet
3. So, eating meat is not morally permissible

I don’t know whether that’s a sound argument, but it is a recognisably philosophical argument, and an interesting one. The following is not, however, a particularly good or interesting argument.

1. Eating meat is morally permissible only if it is necessary for a healthy diet
2. Intuitively, eating meat is not necessary for a healty diet
3. So, eating meat is not morally permissible

If Singer had presented that argument, he wouldn’t have been listened to. The crucial evidence for premise 2 is from nutrition science, not intuition. So it’s not always true that we can replace _p_ with _Intuitively, p_ in philosophical arguments without loss of argumentative force. So it’s not always true that making this replacement merely makes more explicit our underlying reasoning. So it’s not always true that our philosophical evidence consists in intuitions.

All that shows is that not all philosophical evidence consists of intuitions. It doesn’t show that, for example, no philosophical evidence consists of intuitions. One way to ‘psychologise evidence’ is to say that some of the time, or perhaps even frequently, intuitions are a part of our philosophical evidence. If this is plausible, then the interesting question seems to be scalar rather than on/off. It isn’t whether evidence is always intuitions or always something else; it’s how often does evidence consist of intuitions?

There’s a second scalar question we might wonder about, one triggered by the observation that the two positions on evidence we’ve been taking are not strictly speaking incompatible. We could hold, indeed a sensible proponent of the external view probably will hold, that psychological states are among our evidence, even when our evidence also consists in external facts. So in the perceptual case, my evidence might include _both_ that there is a table in front of me, and that I’m forming a visual representation of the table.

When this is true, there are a number of questions we might start to think about concerning the balance between the psychological evidence and the external evidence. For purposes of these posts, what I’ll be interested in is the following question. In such cases, how well does the purely psychological evidence support the conclusion? And in particular, how much better supported is the conclusion by external evidence than it would be by psychological evidence alone? If the answers to those questions are _quite a bit_ and _not a lot_, then the psychological view seems to have turned out to be correct in crucial respects.

Note this doesn’t mean that the psychological states have to do a lot of work. Consider again the position that I get both external and psychological evidence when I look at a table. One might hold (a) the psychological evidence I get provides very strong grounds for believing that there’s a table, while (b) I don’t actually need that evidence, since I have even stronger evidence available, namely that there is a table. That still feels like a case where the person who takes philosophical evidence to be psychological isn’t badly mistaken, even if in fact the psychological evidence in the case doesn’t need to do any work. If (a) were false, and the psychological evidence provided very little by way of justification, then the psychological view does feel like it is mistaken, even if strictly speaking, my psychological states are (weak) evidence that I actually have.

So here’s a somewhat more precise way to say what it is to psychologise evidence in philosophy: it’s to hold that in a large percentage of philosophical argument, a large amount of the evidence consists of intuitions, and by that we mean that intuitions alone can bear a lot of the weight of the philosophical argument even in the absence of non-psychological evidence. In the next post I’ll work through some of the things Williamson says to see how close that position is to his target, and then in the posts to follow, I’ll sketch a defence of such a position.