On the physical side, an illusion of sense, like a just perception, is the result of a fusion of the nervous process answering to a sensation with a nervous process answering to a mental image.

In the case of passive illusions, this fusion may be said to take place in consequence of some point of connection between the two.

The existence of such a connection appears to be involved in the very fact of suggestion, and may be said to be the organic result of frequent conjunctions of the two parts of the nervous operation in our past history.

In the case of active illusions, however, which spring rather from the independent energy of a particular mode of the imagination, this point of organic connection is not the only or even the main thing.

In many cases, as we shall see, there is only a faint shade of resemblance between the present impression and the mental image with which it is overlaid.