The more powerful this disposition in the centres involved in the act of imagination, the less the additional force of external stimulus required to excite them to full activity.
Considering the first division, passive illusions, a little further, we shall see that they may be broken up into two sub-classes, according to the causes of the errors.

In a general way we assume that the impression always answers to some quality of the object which is perceived, and varies with this; that, for example, our sensation of colour invariably represents the quality of external colour which we attribute to the object.

Or, to express it physically, we assume that the external force acting on the sense-organ invariably produces the same effect, and that the effect always varies with the external cause.

But this assumption, though true in the main, is not perfectly correct.

It supposes that the organic conditions are constant, and that the organic process faithfully reflects the external operation.