Ilya Ilyich Mechnikov is a famous Russian scientist, biologist, bacteriologist, one of the founders of comparative pathology, evolutionary embryology, immunology, Nobel Prize laureate in the field of physiology and medicine, academician. Mechnikov believed that with the help of science and culture, man is able to overcome contradictions human nature, to ensure a happy existence based on the feeling of the fullness of life in all its manifestations, and, with the natural transition of the “life instinct” into the “death instinct,” to achieve a fearless end, which is accompanied by a natural desire for death. These views are presented in the books “Studies on Human Nature” and “Studies on Optimism.” Both works comprehensively examine questions about human nature, the nature of aging and death, in addition, a lot of space is given to the theory of evolution, questions of the birth, aging and dying of living organisms. The scientist believed that aging and death occur prematurely in humans, and a special role microbes of the intestinal flora play a role in this, poisoning the body with their toxins, therefore he believed that old age can be treated with diet, preventive and hygienic measures, like any disease. Studying the mechanism of aging, Mechnikov relied not only on his own observations, but also on theories and experiments colleagues, outstanding scientists and physicians. The book contains many studies, theories, opinions, examples of operations, treatment methods - at the same time it is an excursion into the history of medicine, pharmacology, biology, microbiology, bacteriology. Written more than 100 years ago, the book still attracts the attention of a wide range of readers.