As we turn from the objective to the subjective conditions of deception, we enter the true domain of psychology; for the most scientific deceiver is he who employs least external aids, and counts most upon his power of captivating the intellect. Just as we interpret appearances by the forms they most commonly assume, so it is our average normal selves that interpret them. A variation in our sense-organs or in our judging powers will lead to illusion. The effects of contrast may serve as apt illustrations. When passing from a dark to a light room the light seems glaringly bright; a hand immersed in hot water and then in lukewarm water will feel the latter as cold; when accustomed to the silence of the country the bustle of the city seems unusually noisy. Fatigue produces similar results. Fatigue the eye for red, and it sees white light as green; the last mile of a long walk seems the longest; the last hour of a long wait, the most tedious. So long as we recognize our unusual condition and allow for its effects, we are not deceived; but under the influence of emotion this power is readily lost, as it may be permanently lost in the insane. The delusions of the insane are often influenced by misinterpretations of real but abnormal sensations under the guidance of a dominant idea. On the basis of an anæsthetic skin a patient may come to believe that he is made of glass or stone; subjective noises in the ear, due to disturbances of the circulation, are transformed into the jeers and taunts of an invisible persecutor. But for the present we will assume that the judging powers do not vary beyond their normal limits.