But he only just raised the veil of the great mystery; it was his last work.... * * * * * Metchnikoff's philosophical evolution ran on parallel lines with his scientific researches. When studying the laws and the unity of vital phenomena he found that their harmony was occasionally broken by the collision of internal conditions with the environment and that regrettable consequences ensued. He saw an example of that in human nature, full of disharmonies due to its animal origin. These considerations caused the pessimism of his youth. But his energetic, pugnacious temperament could not remain content with a passive acceptance of facts. He started to study the lack of harmony in human nature and its causes, and sought for means to combat these causes. Gradually he reached the conclusion that the greatest human disharmonies are provoked by the rupture of the normal cycle of our life, by the precocity of senility and of death, chiefly arising from a chronic poisoning by the toxins of intestinal microbes. But having acquired the conviction that it is possible to struggle against that intoxication, he concluded that science, which has already done so much to fight diseases, would also find means of struggling against _premature_ old age and _precocious_ death, thus leading us to the normal vital cycle, _orthobiosis_. Then disharmony, transformed into harmony, will cause the greatest of ills to disappear. Faith in the power of Science and in the possibility of modifying human nature itself through Science was the foundation of the optimistic philosophy of his maturity. Thoughts full of strength and hope shine like leading stars all along his philosophical works. "Alone, Rational Science is capable of showing humanity the true path." "The real goal of human existence consists in an active life in conformity with individual capacity; in a life prolonged until the appearance of the _death-instinct_, and until Man, satisfied with the duration of his existence, feels the desire for annihilation." "Man is capable of great works; that is why it is desirable that he should modify human nature and transform its disharmonies into harmonies." "If an ideal capable of uniting _men_ in a sort of religion is possible, it can only be founded on scientific principles. And, if it is true, as is often affirmed, that man cannot live without faith, it must be faith in the power of Science." Thus Elie Metchnikoff had begun by the study of nascent life in inferior beings; by a logical and continuous chain, he had followed the whole cycle of development of living beings in their continuity and their whole. From the initial question of intracellular digestion he had reached the most exalted problems which can occupy our minds, the harmonising of human discords through knowledge and will. Such is the harmonious edifice which he has built.