Hitherto we have spoken of single words, and among these we have considered only those that can naturally be employed singly. A child uses single words of a certain kind before constructing sentences; but some words presuppose sentences. No one would use the word “paternity” until after using such sentences as “John is the father of James”; no one would use the word “causality” until after using such sentences as “the fire makes me warm”. Sentences introduce new considerations, and are not quite so easily explained on behaviourist lines. Philosophy, however, imperatively demands an understanding of sentences, and we must therefore consider them. As we found earlier, all infants outside Patagonia begin with single words, and only achieve sentences later. But they differ enormously in the speed with which they advance from the one to the other. My own two children adopted entirely different methods. My son first practised single letters, then single words, and only achieved correct sentences of more than three or four words at the age of two and three months. My daughter, on the contrary, advanced very quickly to sentences, in which there was hardly ever an error. At the age of eighteen months, when supposed to be sleeping, she was overheard saying to herself: “Last year I used to dive off the diving-board, I _did_.” Of course “last year” was merely a phrase repeated without understanding. And no doubt the first sentences used by children are always repetitions, unchanged, of sentences they have heard used by others. Such cases raise no new principle not involved in the learning of words. What does raise a new principle is the power of putting together known words into a sentence which has never been heard, but which expresses correctly what the infant wishes to convey. This involves the power to manipulate form and structure. It does not of course involve the apprehension of form or structure in the abstract, any more than the use of the word “man” involves apprehension of a universal. But it does involve a causal connection between the form of the stimulus and the form of the reaction. An infant very soon learns to be differently affected by the statement “cats eat mice” from the way he would be affected by the statement “mice eat cats”; and not much later he learns to make one of these statements rather than the other. In such a case, the cause (in hearing) or the effect (in speaking) is a whole sentence. It may be that one part of the environment is sufficient to cause one word, while another is sufficient to cause another, but it is only the two parts in their relation that can cause the whole sentence. Thus wherever sentences come in we have a causal relation between two complex facts, namely the fact asserted and the sentence asserting it; the facts as wholes enter into the cause-and-effect relation, which cannot be explained wholly as compounded of relations between their parts.