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. When I say that “inferences” are involved, I am saying something not quite accurate unless carefully interpreted. In “seeing a chair”, for instance, we do not first apprehend a coloured pattern, and then proceed to infer a chair: belief in the chair arises spontaneously when we see the coloured pattern. But this belief has causes not only in the present physical stimulus, but also partly in past experience, partly in reflexes. In animals, reflexes play a very large part; in human beings, experience is more important. The infant learns slowly to correlate touch and sight, and to expect others to see what he sees. The habits which are thus formed are essential to our adult notion of an object such as a chair. The perception of a chair by means of sight has a physical stimulus which affects only sight directly, but stimulates ideas of solidity and so on through early experience. The inference might be called “physiological”. An inference of this sort is evidence of past correlations, for instance between touch and sight, but may be mistaken in the present instance; you may, for instance, mistake a reflection in a large mirror for another room. Similarly in dreams we make mistaken physiological inferences. We cannot therefore feel certainty in regard to things which are in this sense inferred, because, when we try to accept as many of them as possible, we are nevertheless compelled to reject some for the sake of self-consistency.