'Instinct is an innate psycho-physical tendency to pay attention to objects of a certain class, to experience emotional excitement of peculiar quality on such perceptions, and to act or have an impulse to act in a particular way with regard to that object.' We can see that instinct suffices for the bird or insect, living almost entirely in the unconscious, to carry on the important affairs of life. Even in regard to what looks like the exercise of reason or memory we can find a parallel in the human unconscious. The unreasonable fears and obsessions of the 'shell-shocked' soldier rest upon causes of which he is unaware, and the burnt child dreads the fire even if he were too young to remember the burning. The chicken that has once tasted a nauseous caterpillar will ever after avoid its like, but we only know that a certain emotion is called up by the sight of the caterpillar which causes the chicken to abstain; it is an unnecessary assumption that memory, as we know it, is concerned. The obsession of the soldier who felt that he must attack his companion (see Chapter VIII) arose from the unconscious, and those animal actions which we attribute to memory can similarly have their origins apart from consciousness. McDougall's definition of instinct applies very well to obsessions, except that the latter are not innate but acquired; that one definition should apply to both groups is due to them all having their origin in the unconscious. Man, though urged by the instincts and memories of his unconscious, yet lives in his stream of consciousness and tends to believe that there is no other mind-work involved in his thoughts and actions; but as the latest evolved function is the most variable and unstable so man's consciousness is his most uncertain function, its chief variability being in the extent to which it controls or is controlled by the unconscious. The ideal human mind would be perfectly integrated, there would be no logic-tight compartments, all its complexes would be apparent to the consciousness, all memories available when needed, all emotions assigned to their proper cause and all instincts recognised and well-directed; and the owner of it would find life in our world intolerable. Remote from this ideal is the mind whose unconscious has taken the place, wholly or in part, of the stream of consciousness. Perhaps the consciousness has not developed--then we find idiocy or imbecility; perhaps some distorted emotion from the unconscious has been the source of a dissociated stream of ideas which becomes predominant and brings its owner within the legal definition of a lunatic.