Philosophy seems, then, to be a continuation of that process of detecting illusion with which science in part concerns itself. Indeed, it is evident that our special study has a very close connection with the philosophic inquiry. What philosophy wants is something intuitively certain as its starting-point, some _point d'appui_ for its construction. The errors incident to the process of reasoning do not greatly trouble it, since these can, in general, be guarded against by the rules of logic. But error in the midst of what, on the face of it, looks like intuitive knowledge naturally raises the question, Is there any kind of absolutely certain cognition, any organ for the accurate perception of truth? And this intimate relation between the scientific and the philosophic consideration of illusion is abundantly illustrated in the history of philosophy. The errors of sense, appearing in a region which to the vulgar seems so indubitable, have again and again set men thinking on the question, "What is the whole range of illusion? Is perception, as popularly understood, after all, a big hallucination? Is our life a dream?"[155] On the other hand, if our study of the wide range of illusion is fitted to induce that temper of mind which is said to be the beginning of philosophy, that attitude of universal doubt expressed by Descartes in his famous maxim, _De omnibus dubitandum_, a consideration of the process of correction is fitted to lead the mind on to the determination of the conditions of accurate knowledge. It is evident, indeed, that the very conception of an illusion implies a criterion of certainty: to call a thing illusory, is to judge it by reference to some accepted standard of truth.