The illusions dependent on vivid, expectation thus answer much less to an objective conjunction of past experiences than to a capricious subjective conjunction of mental images.

Here, then, the fusion of nervous processes must have another cause.

And it is not difficult to assign such a cause.

The antecedent activity of imagination doubtless involves as its organic result a powerful temporary disposition in the nervous structures concerned to go on acting.

In other words, they remain in a state of sub-excitation, which can be raised to full excitation by a slight additional force.