Without doubt, many of our efforts of thinking miscarry, not only in the sciences, as is amply proved by their history, but also in ordinary life. But in the simple cases of mistake which are, after all, the commonest, the correct thought so rapidly replaces the incorrect, that the latter is never, or rarely, noticed. Kant, in his theory of the "synthetic unity of apperception," had an inkling of this activity of thought in the systematic organisation of the world-content, as we have here developed it. But his failure to appreciate clearly the real function of thinking is revealed by the fact, that he believes himself able to deduce the a priori laws of Pure Natural Science from the rules according to which this synthetic activity proceeds. Kant has overlooked that the synthetic activity of thinking is merely the preparation for the discovery of natural laws properly so-called. Suppose we select two items, a and b, from the Given. For knowledge to arise of a nexus according to law between a and b, the first requirement is that thinking should so relate a and b, that the relation may appear to us as given. Thus, the content proper of the law of nature is derived from what is given, and the sole function of thinking is to establish such relations between the items of the world-picture that the laws to which they are subject become manifest. The pure synthetic activity of thinking is not the source of any objective laws whatever. We must inquire what part thinking plays in the formation of our scientific world-picture as distinct from the merely given one. It follows from our account that thinking supplies the formal principle of the conformity of phenomena to law. Suppose, in our example above, that a is the cause, b the effect. Unless thinking were able to produce the concept of causality, we should never be able to know that a and b were causally connected. But, in order that we may know, in the given case, that a is the cause and b the effect, it is necessary for a and b to possess the characteristics which we mean when we speak of cause and effect. A similar analysis applies to the other categories of thought.