§ 5. In practice, however, it constantly happens, as a consequence of the fragmentary way in which our experiences come to us, that comprehensiveness and thorough-going systematic unity seem to be opposed to one another. Thus we can see, as a general principle, that the systematic organisation of knowledge depends upon its extent. The wider our knowledge, the greater on the whole the degree to which it exhibits organic structure; the systematisation of science and its extension ultimately go together. Yet at any one moment in the development of knowledge the recognition of fresh truths may necessitate a temporary introduction of disorganisation and discrepancy among the accepted principles of science. Thus in the history of geometry the recognised principles of the science were temporarily disorganised by the admission of incommensurable magnitudes which was forced upon the early Greek mathematicians by the discovery that the side and diagonal of a square have no common measure, and the discrepancy was only removed when it became possible to revise the principles of the theory of numbers itself. So again at the present day there is a real danger that premature anxiety to give the study of Psychology precise systematic character by an exact definition of its subject and its relation to the various physical and mental sciences, may stand in the way of the extension of our knowledge of the facts of psychical life. We have often to purchase an important extension of knowledge at the cost of temporary confusion of principles, and to be content to wait for the future readjustment of facts to principles in the course of subsequent progress.