First of all, illusion varies very considerably in its degree of force and persistence.

Thus, in general, a presentative illusion is more coercive than a representative; an apparent reality present to the mind is naturally felt to be more indubitable than one absent and only represented.

On the other hand, a representative illusion is often more enduring than a presentative, that is to say, less easily found out.

It is to be added that a good deal of illusion is only partial, there being throughout an under-current of rational consciousness, a gentle play of self-criticism, which keeps the error from developing into a perfect self-delusion.

This remark applies not only to the innocent illusions of art, but also to many of our every-day illusions, both presentative and representative.