And while illusion has thus so great a depth in the individual mind, it has a no less striking breadth or extent in the collective human mind.

No doubt its grosser forms manifest themselves most conspicuously in the undisciplined mind of the savage and the rustic; yet even the cultivated mind is by no means free from its control.

In truth, most of the illusions illustrated in this work are such as can be shared in by all classes of mind.
In view of this wide far-reaching area of ascertained error, the mind naturally asks, What are the real limits of illusory cognition, and how can we be ever sure of having got beyond them?
This question leads us on to philosophical problems of the greatest consequence, problems which can only be very lightly touched in this place.

Before approaching these, let us look back a little more carefully and gather up our results, reflect on the method which we have been unconsciously adopting, and inquire how far this scientific mode of procedure will take us in determining what is the whole range of illusory cognition.
We have found an ingredient of illusion mixed up with all the popularly recognized forms of immediate knowledge.

Yet this ingredient is not equally conspicuous in all cases.