rate
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.

The mental processes involved in detecting, resisting, and overcoming
illusion, are a very interesting subject for the psychologist, though we
have not space here to investigate them fully. Turning to presentative,
and more particularly sense-illusions, we find that the detection of an
illusion takes place now by an appeal from one sense to another, for
example, from sight to touch, by way of verification;[156] now (as in
Myer's experiment) by a reference from sense and presentation altogether
to representation or remembered experience and a process of reasoning;
and now, (as in the illusions of art) conversely, by a transition of
mind from what is suggested to the actual sense-impression of the
moment. In the sphere of memory, again, illusion is determined, as such,
now by attending more carefully to the contents of memory, now by a
process of reasoning from some presentative cognition. Finally, errors
in our comprehensive general representations of things are known to be
such partly by reasoning from other conceptions, and partly by a
continual process of reduction of representation to presentation, the
general to the particular. I may add that the correction of illusion by
an act of reflection and reasoning, which brings the part into
consistent relation with the whole of experience, includes throughout
the comparison of the individual with the collective or social
experience.[157]

We may, perhaps,