It is well known that a man who has lost a leg refers all sensations arising from a stimulation of the truncated fibres to his lost foot, and in some cases has even to convince himself of the non-existence of his lost member by sight or touch.

Patients often describe these experiences in very odd language.

"If," says one of Dr.
Weir Mitchell's patients, "I should say I am more sure of the leg which ain't than the one which air, I guess I should be about correct."[30] There is good reason for supposing that this source of error plays a prominent part in the illusions of the insane.

Diseased centres may be accompanied by disordered peripheral structures, and so subjective sensation may frequently be the starting-point of the wildest illusions.
Thus, a patient's horror of poison may have its first origin in some subjective gustatory sensation.

Similarly, subjective tactual sensations may give rise to gross illusions, as when a patient "feels" his body attacked by foul and destructive creatures.
It may be well to remark that this mistaken interpretation of the seat or origin of subjective sensation is closely related to hallucination.
In so far as the error involves the ascription of the sensation to a force external to the sense-organ, this part of the mental process must, when there is no such force present, be viewed as hallucinatory.