word or see a written word, because each is a physical occurrence. If I say to you, "mind is different from matter", there may be only a very slight resemblance between the thought that I am trying to express and the thought which is aroused in you, but these two thoughts have just this in common, that they can be expressed by the same words. Similarly, there may be great differences between what you and I see when, as we say, we look at the same chair; nevertheless we can both express our perceptions by the same words.
A thought and a perception are thus not so very different in their own nature. If physics is true, they are different in their correlations: when I see a chair, others have more or less similar perceptions, and it is thought that these are all connected with light-waves coming from the chair, whereas, when I think a thought, others may not be thinking anything similar. But this applies also to feeling a toothache, which would not usually be regarded as a case of introspection. On the whole, therefore, there seems no reason to regard introspection as a different kind of knowledge from external perception. But this whole question will concern us again at a later stage.
As for the trustworthiness of introspection, there is again a complete parallelism with the case of external perception. The actual datum, in each case, is unimpeachable, but the extensions which we make instinctively are questionable. Instead of saying, "I am believing that mind is different from matter", you ought to say, "certain images are occurring in a certain relation to each other, accompanied by a certain feeling". No words exist for describing the actual occurrence in all its particularity; all words, even proper names, are general, with the possible exception of "this", which is ambiguous. When you translate the occurrence into words, you are making generalisations and inferences, just as you are when you say "there is a chair". There is really no vital difference between the two cases. In each case, what is really a datum is unutterable, and what can be put into words involves inferences which may be mistaken.
