III. In the same way we must regard the third stage, the unconscious memory of those animals which have a nervous system, as a reproduction of the corresponding "unconscious presentations" which are stored up in certain ganglionic cells. In most of the lower animals all memory is unconscious. Moreover, even in man and the higher animals, to whom we must ascribe consciousness, the daily acts of unconscious memory are much more numerous and varied than those of the conscious faculty; we shall easily convince ourselves of that if we make an impartial study of a thousand unconscious acts we perform daily out of habit, and without thinking of them, in walking, speaking, writing, eating, and so forth. IV. Conscious memory, which is the work of certain brain-cells in man and the higher animals, is an "internal mirroring" of very late development, the highest outcome of the same psychic reproduction of presentations which were mere unconscious processes in the ganglionic cells of our lower animal ancestors. The concatenation of presentations--usually called the association of ideas--also runs through a long scale, from the lowest to the highest stages. This, too, is originally and predominantly unconscious ("instinct"); only in the higher classes of animals does it gradually become conscious ("reason"). The psychic results of this "association of ideas" are extremely varied; still, a very long, unbroken line of gradual development connects the simplest unconscious association of the lowest protist with the elaborate conscious chain of ideas of the civilized man. The _unity of consciousness_ in man is given as its highest consequence (Hume, Condillac). All higher mental activity becomes more perfect in proportion as the normal association extends to more numerous presentations, and in proportion to the order which is imposed on them by the "criticism of pure reason." In dreams, where this criticism is absent, the association of the reproduced impressions often takes the wildest forms. Even in the work of the poetic imagination, which constructs new groups of images by varying the association of the impressions received, and in hallucinations, etc., they are often most unnaturally arranged, and seem to the prosaic observer to be perfectly irrational. This is especially true of supernatural "forms of belief," the apparitions of spiritism, and the fantastic notions of the transcendental dualist philosophy; though it is precisely these _abnormal associations_ of "faith" and of "revelation" that have often been deemed the greatest treasures of the human mind (cf. chap. xvi.).