A great deal of the confusion about relations which has prevailed in practically all philosophies comes from the fact, which we noticed just now, that relations are indicated, not by other relations, but by words which, in themselves, are just like other words. Consequently, in thinking about relations, we constantly hover between the unsubstantiality of the relation itself and the substantiality of the word. Take, say, the fact that lightning precedes thunder. If we were to express this by a language closely reproducing the structure of the fact, we should have to say simply: “lightning, thunder”, where the fact that the first word precedes the second means that what the first word means precedes what the second word means. But even if we adopted this method for temporal order, we should still need words for all other relations, because we could not without intolerable ambiguity symbolise them also by the order of our words. All this will be important to remember when we come to consider the structure of the world, since nothing but a preliminary study of language will preserve us from being misled by language in our metaphysical speculations. Throughout this chapter I have said nothing about the narrative and imaginative uses of words; I have dealt with words in connection with an immediate sensible stimulus closely connected with what they mean. The other uses of words are difficult to discuss until we have considered memory and imagination. In the present chapter I have confined myself to a behaviouristic explanation of the effects of words heard as stimuli, and the causes of words spoken when the words apply to something sensibly present. I think we shall find that other uses of words, such as the narrative and imaginative, involve only new applications of the law of association. But we cannot develop this theme until we have discussed several further psychological questions. CHAPTER V PERCEPTION OBJECTIVELY REGARDED It will be remembered that the task upon which we are at present engaged is the definition of “knowledge” as a phenomenon discoverable by an outside observer. When we have said what we can from this objective standpoint, we will ask ourselves whether anything further, and if so what, is to be learnt from the subjective standpoint, in which we take account of facts which can only be discovered when the observer and the observed are the same person. But for the present we will resolutely confine ourselves to those facts about a human being which another human being can observe, together with such inferences as can be drawn from these facts.