Identifying the necessary and sufficient features of life has a long tradition in theoretical biology. But living systems are products of evolution, and an answer in very general terms, even if possible, is likely to remain purely phenomenological: going deeper into mechanisms means having to account for the organization of various processes, and such organization has been realized in several different ways by evolution. Eukaryotic cells (such as those from which we are made) are much more complicated than prokaryotes (such as bacteria), and eukaryotes harbour organelles that were once free-living bacteria. A further complication is that multicellular organisms consist of building blocks — cells — that are also alive. So aiming for a general model of all kinds of living beings would be fruitless; instead, such models have to be tied to particular levels of biological organization.