Thus, the retina, in ordinary circumstances, is stimulated by light.

Owing to this fact, there has arisen a deeply organized habit of translating the impression in one particular way.

Thus, I instinctively regard a sensation received by means of the optic nerve as one caused by light.
Accordingly, whenever circumstances arise in which a like sensation is produced by another kind of stimulus, we fall into illusion.

The phosphenes, or circles of light which are seen when the hinder part of the eyeball is pressed, may be said to be illusory in so far as we speak of them as perceptions of light, thus referring them to the external physical agency which usually causes them.

The same remark applies to those "subjective sensations," as they are called, which are known to have as their physical cause subjective stimuli, consisting, in the case of sight, in varying conditions of the peripheral organ, as increased blood-pressure.