The degree of illusion increases in proportion as the imaginative element gains in force relatively to the present impression; till, in the wild illusions of the insane, the amount of actual impression becomes evanescent.

When this point is reached, the act of imagination shows itself as a purely creative process, or an hallucination.
While we may thus trace the progress of illusion towards hallucination by means of the gradual increase in force and extent of the imaginative, or indirect, as opposed to the sensuous, or direct, element in perception, we have found a second starting-point for this movement in the mechanism of sensation, involving, as it does, the occasional production of "subjective sensations." Such sensations constitute a border-land between the regions of illusion in the narrow sense, and hallucination.

In their simplest and least developed form they may be regarded, at least in the case of hearing and sight, as partly hallucinatory; and they serve as a natural basis for the construction of complete hallucinations, or hallucinatory percepts.
In these different ways, then, the slight, scarcely noticeable illusions of normal life lead up to the most startling hallucinations of abnormal life.

From the two poles of the higher centres of attention and imagination on the one side, and the lower regions of nervous action involved in sensation on the other side, issue forces which may, under certain circumstances, develop into full hallucinatory percepts.

Thus closely is healthy attached to morbid mental life.