I shall first speak of _Rationalism_, which I may somewhat loosely define as the doctrine that the reason can attain truths independently of observation--can go beyond experienced fact and the deductions which experience seems to justify us in making from experienced fact. The definition cannot mean much to us until it is interpreted by a concrete example, and I shall turn to such. It must, however, be borne in mind that the word "rationalism" is meant to cover a great variety of opinions, and we have said comparatively little about him when we have called a man a rationalist in philosophy. Men may agree in believing that the reason can go beyond experienced fact, and yet may differ regarding the particular truths which may be thus attained. Now, when Descartes found himself discontented with the philosophy that he and others had inherited from the Middle Ages, and undertook a reconstruction, he found it necessary to throw over a vast amount of what had passed as truth, if only with a view to building up again upon a firmer foundation. It appeared to him that much was uncritically accepted as true in philosophy and in the sciences which a little reflection revealed to be either false or highly doubtful. Accordingly, he decided to clear the ground by a sweeping doubt, and to begin his task quite independently. In accordance with this principle, he rejected the testimony of the senses touching the existence of a world of external things. Do not the senses sometimes deceive us? And, since men seem to be liable to error in their reasonings, even in a field so secure as that of mathematical demonstration, he resolved further to repudiate all the reasonings he had heretofore accepted. He would not even assume himself to be in his right mind and awake; might he not be the victim of a diseased fancy, or a man deluded by dreams? Could anything whatever escape this all-devouring doubt? One truth seemed unshakable: his own existence, at least, emerged from this sea of uncertainties. I may be deceived in thinking that there is an external world, and that I am awake and really perceive things; but I surely cannot be deceived unless I exist. _Cogito, ergo sum_--I think, hence I exist; this truth Descartes accepted as the first principle of the new and sounder philosophy which he sought. As we read farther in Descartes we discover that he takes back again a great many of those things that he had at the outset rejected as uncertain. Thus, he accepts an external world of material things. How does he establish its existence? He cannot do it as the empiricist does it, by a reference to experienced fact, for he does not believe that the external world is directly given in our experience. He thinks we are directly conscious only of our _ideas_ of it, and must somehow prove that it exists over against our ideas.