Definition: Living substance is a kind of relatively steady and complex system with special structure, the structure enables it to have the function of making its structure survive in nature by self-reproduction and other positive feedback. The ability to make its complex structure survive by its own properties is vitality (life).

A lot of non-living things are able to survive steadily for a long time, but that depends on environment instead of properties of themselves. Some non-living systems have properties of positive feedback, for instance, laser, Benard instability, Belousov-Zhabotinskii reaction. But these structures of positive feedback are extremely unsteady, which is difficult to emerge and easy to break in nature. So they are not life.

There were more than 1 billion years before life emerged after the Earth had formed. What condition was absent for life origin during those years? All conditions had existed including natural selection, variation, fluctuation, openness, negative entropy and self- organization, except self-reproduction. The history of life began once the ability of self-reproduction emerge.

Ⅶ. Artificial Life

Research of artificial life has progressed greatly since 1987 when the first International Conference of Artificial Life was held. Artificial life, in fact, is nothing else than a kind of computer programs which are able to show some characteristics of life. The research of artificial life enlightens that it is possible for life to exist without components of carbon, hydrogen, oxygen, phosphorous and so on. Therefore, chemical ways is not necessary conditions of the origin of life.

Like molecular biology, though research of artificial life has progressed greatly it has not provided a precise answer of definition and essence of life finally. Though practice may promote the development of theory, it is not able to substitute theory, and needs guidance of theory. As products of man, some properties of artificial life can be added or eliminated, in order to prove whether these properties are sufficient and necessary conditions for their living. Experts of artificial life will do this kind of experiments, if they are aware that it is important to search sufficient and necessary conditions of life to solve the puzzle of life. It will be discovered by these experiments that none of characteristics, including metabolism, growth, evolution, adaptation and so on, is the sufficient or necessary conditions of the origin of life, and only self-reproduction is the sufficient and necessary condition of the origin of life.