Our brief reference to some of the principal inquiries of philosophy shows that it tends to throw doubt on things which the unreflecting popular mind holds to be indubitable.  Different schools of philosophy have shown themselves unequally concerned about these so-called intuitive certainties.  In general it may be said that philosophy, though, as I have remarked, theoretically free to set up its own standard of certainty, has in practice endeavoured to give a meaning to, and to find a justification for the assumptions or first principles of science.  On the other hand, it has not hesitated, when occasion required, to make very light of the intuitive beliefs of the popular mind as interpreted by itself.  Thus, rationalists of the Platonic type have not shrunk from pronouncing individual impressions and objects illusory, an assertion which certainly seems to be opposed to the assumptions of common sense, if not to those of science.  On the other hand, the modern empirical or association school is quite ready to declare that the vulgar belief in an external world, so far as it represents this as independent of mind,[154] is an illusion; that the so-called necessary beliefs respecting identity, uniformity, causation, etc., are not, strictly speaking, necessary; and so on.  And in these ways it certainly seems to come into conflict with popular convictions, or intuitive certainties, as they present themselves to the unreflecting intelligence.

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.