We thus come back to our starting-point.  While there are close relations, psychological and logical, between the scientific study of the ascertained facts of illusion and the philosophic determination of what is illusory in knowledge as a whole, the two domains must be clearly distinguished.  On purely scientific ground we cannot answer the question, “How far does illusion extend?” The solution of this question must be handed over to the philosopher, as one aspect of his problem of cognition.

One or two remarks may, perhaps, be hazarded in concluding this account of the relation of the scientific to the philosophic problem of illusion.  Science, as we have seen, takes its stand on a stable consensus, a body of commonly accepted belief.  And this being so, it would seem to follow, that so far as she is allowed to interest herself in philosophic questions, she will naturally be disposed to ask, What beliefs are shared in by all minds, so far as normal and developed?  In other words, she will be inclined to look at universality as the main thing to be determined in the region of philosophic inquiry.  The metaphysical sceptic, fond of daring exploits, may break up as many accepted ideas as he likes into illusory debris, provided only he has some bit of reality left to take his stand on.  Meanwhile, the scientific mind, here agreeing with the practical mind, will ask, “Will the beliefs thus said to be capable of being shown to be illusory ever cease to exercise their hold on men’s minds, including that of the iconoclast himself?  Is the mode of demonstration of such a kind as to be likely ever to materially weaken the common-sense ’intuition’?”

This question would seem to be most directly answerable by an appeal to individual testimony.  Viewed in this light, it is a question for the present, for some few already allege that in their case philosophic reasonings exercise an appreciable effect on these beliefs.  And so far as this is so, the man of scientific temper will feel that there is a question for him.

It is evident, however, that the question of the persistence of these fundamental beliefs is much more one for the future than for the present.  The correction of a clearly detected illusion is, as I have more than once remarked, a slow process.  An illusion such as the apparent movement of the sun will persist as a partially developed error long after it has been convicted.  And it may be that the fundamental beliefs here referred to, even if presumably illusory, are destined to exercise their spell for long ages yet.

Whether this will be the case or not, whether these intuitive beliefs are destined slowly to decay and be dissolved as time rolls on, or whether they will retain an eternal youth, is a question which we of to-day seem, on a first view of the matter, to have no way of answering which does not assume the very point in question—­the truth or falsity of the belief.  This much may, however, be said.