He considered "old age" as a pathological phenomenon. He saw in it one of the most important disharmonies of human nature, because of the fact that neither senility nor death is accompanied by a natural instinct. The accomplishment of every physiological function leads to satiety or to a desire for rest; after a busy day, man feels an instinctive need for rest and sleep. But, in his maturity, he has no desire to grow old, and in his old age none to die. It is rare that one should aspire to die, and nobody wishes to grow old. These facts are in contradiction with other natural phenomena; they are all the more discordant that they play an immense part in our psychical life. After a general review of opinions on human nature, Metchnikoff analysed it from the biological point of view; he revealed its discords and concluded that it is far from being perfect. In his eyes, the lack of harmony in the human being is an inheritance from our animal ancestors; they have handed down to us a whole series of remains of organs which are not only useless but even harmful in the new conditions of human existence. The large intestine, inherited from mammalian ancestors, holds the first place among those noxious organs. This reservoir of food refuse was very useful to our animal forebears in their struggle for existence; it allowed them not to interrupt their flight whilst pursued by their enemies. In man, whose life conditions are different, a large intestine of that size, without offering the same advantages, is a source of slow and continuous poisoning and a cause of premature senility and death. Man, after acquiring a still higher development, realised these evils and made concentrated efforts to fight them and to soothe his own terrors. It is for that object that the divers religious and philosophical systems were created, in which humanity sought for consolation. Finding none there, man turned to Science, which, at first, neither solved his doubts nor eliminated his sufferings. But Science provided him with rational methods of research, owing to which he gradually progressed and conquered a series of truths, allowing him gradually to struggle against some of his troubles and to solve some of his problems. Science has already done much to diminish the diseases which are among the chief scourges of humanity. It has thrown light upon the causes of many of them and has found preventive and curative remedies for several. Surgery, antiseptics, serotherapy, vaccinations already yield secure results. Hygiene and prophylaxis are in course of development, and a vast prospect is open to them in the future. But our heaviest burdens, senility and death, common to all, have yet scarcely been studied. Having expounded his views on senility and proved that it is a pathological phenomenon, Metchnikoff concluded that to struggle against it was quite as possible as to struggle against disease.