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.  There seems to be no sudden break between our most sober every-day recognitions of familiar objects and the wildest hallucinations of the demented.  As we pass from the former to the latter, we find that there is never any abrupt transition, never any addition of perfectly new elements, but only that the old elements go on combining in ever new proportions.

The connection between the illusory side of our life and insanity may be seen in another way.  All illusion has as its negative condition an interruption of the higher intellectual processes, the due control of our mental representations by reflection and reason.  In the case of passive illusions, the error arises from our inability to subordinate the suggestion made by some feature of the present impression to the result of a fuller inspection of the object before us, or of a wider reflection on the past.  In other words, our minds are dominated by the partial and the particular, to the exclusion of the total or the general.  In active illusions, again, the powers of judgment and reflection, including those of calm perception itself, temporarily vacate their throne in favour of imagination.  And this same suspension of the higher intellectual functions, the stupefaction of judgment and reflection made more complete and permanent, is just what characterizes insanity.

We may, perhaps, express this point of connection between the illusions of normal life and insanity by help of a physiological hypothesis.  If the nervous system has been slowly built up, during the course of human history, into its present complex form, it follows that those nervous structures and connections which have to do with the higher intellectual processes, or which represent the larger and more general relations of our experience, have been most recently evolved.  Consequently, they would be the least deeply organized, and so the least stable; that is to say, the most liable to be thrown hors de combat.  This is what happens temporarily in the case of the sane, when the mind is held fast by an illusion.  And, in states of insanity, we see the process of nervous dissolution beginning with these same nervous structures, and so taking the reverse order of the process of evolution.[67] And thus, we may say that throughout the mental life of the most sane of us, these higher and more delicately balanced structures are constantly in danger of being reduced to that state of inefficiency, which in its full manifestation is mental disease.