Trying to Define Life?

Can we define what life is? This question tormented John Stewart, to the point that he gave one of his books the following title: “Does Life Exist?”. As a biologist like him, I branched off into the didactics and epistemology of biology. Let me first recall that the definitions of what life is evolve both during childhood and during historical periods. John Stewart's contribution is then presented into three points: life is an open thermodynamic system which is auto-poietic and has functions. After summarizing and discussing these three points, I add a fourth which has achieved consensus in the scientific community: it is a system which reproduces itself almost identically. But these attempts to define Life remain questions that are still open. John Stewart's incessant desire not to reduce the living to physico-chemical processes could recall a vitalist position, but the complexity of the living is rather defined for him as that of an autopoietic system capable of emergences, which he illustrated minimally by computer simulations. Finally, a coherence is suggested between the attempts at definitions of life proposed by John Stewart, and his militant positions full of life!