The first class of illusions arises from without, the sense-impression being the starting-point, and the process of preperception being controlled by this.

The second class arises rather from within, from an independent or spontaneous activity of the imagination.

In the one case the mind is comparatively passive; in the other it is active, energetically reacting on the impression, and impatiently anticipating the result of the normal process of preperception.

Hence I shall, for brevity's sake, commonly speak of them as Passive and Active Illusions.[16] I may, perhaps, illustrate these two classes of illusion by the simile of an interpreter poring over an old manuscript.

The first would be due to some peculiarity in the document misleading his judgment, the second to some caprice or preconceived notion in the interpreter's mind.
It is not difficult to define conjecturally the physiological conditions of these two large classes of illusion.