These and other familiar facts prove that man's consciousness--and that of the nearest mammals--is _changeable_, and that its activity is always open to modification from inner (alimentation, circulation, etc.) and outer causes (lesion of the brain, stimulation, etc.). Very instructive, too, are the facts of double and intermittent consciousness, which remind us of "alternate generations of presentations." The same individual has an entirely different consciousness on different days, with a change of circumstances; he does not know to-day what he did yesterday: yesterday he could say, "I am I"; to-day he must say, "I am another being." Such intermittence of consciousness may last not only days, but months, and even years; the change may even become permanent. As everybody knows, the new-born infant has no consciousness. Preyer has shown that it is only developed after the child has begun to speak; for a long time it speaks of itself in the third person. In the important moment when it first pronounces the word "I," when the feeling of self becomes clear, we have the beginning of self-consciousness, and of the antithesis to the non-ego. The rapid and solid progress in knowledge which the child makes in its first ten years, under the care of parents and teachers, and the slower progress of the second decade, until it reaches complete maturity of mind, are intimately connected with a great advancement in the growth and development of consciousness and of its organ, the brain. But even when the pupil has got his "certificate of maturity" his consciousness is still far from mature; it is then that his "world-consciousness" first begins to develop, in his manifold relations with the outer world. Then, in the third decade, we have the full maturity of rational thought and consciousness, which, in cases of normal development, yield their ripe fruits during the next three decades. The slow, gradual degeneration of the higher mental powers, which characterizes senility, usually sets in at the commencement of the seventh decade--sometimes earlier, sometimes later. Memory, receptiveness, and interest in particular objects gradually decay; though productivity, mature consciousness, and philosophic interest in general truths often remain for many years longer. The individual development of consciousness in earlier youth proves the universal validity of the _biogenetic law_; and, indeed, it is still recognizable in many ways during the later years. In any case, the ontogenesis of consciousness makes it perfectly clear that it is not an "immaterial entity," but a physiological function of the brain, and that it is, consequently, no exception to the general law of substance.