"Molecular biologists usually characterize life by objective properties like self-reproduction, variability and selection, adaptation, and metabolism. These characteristics do not constitute the definition of life but are properties of what we have previously recognized as living (on the Earth) on intuitive and subjective grounds. I think there is no objective definition of life (especially of extraterrestrial life) but only a subjective definition: we will decide that a system is 'living' when we will have relations with it (essentially based on communication) which will appear to us sufficiently rich, interesting, complex. Since any type of communication needs a physical support, the system considered must itself have (or be) a complex physical structure. The living systems on Earth have a hierarchical structure of the following type: living system ... organs ... cells ... 'central memory' (DNA). The transition from one stage to another in this scheme may be contingent and have no logical necessity. One could treat the problem at any of these levels. But molecular biologists are used to building the macroscopic from the microscopic. Since there is no other well established working method, I will thus restrict myself to the aim of finding a model of 'central memory.' It will be required to have the fundamental characteristics of DNA: to be stable, to contain rich information, and to diffuse, thanks for a reading mechanism, this information into the surrounding medium (protein synthesis in the case of the cell). These will be the minimal characteristics of ... model for a crystalline physiology. The places of possible occurrence in nature of this kind of physiology, terrestrial and extraterrestrial rocks, interplanetary dust, white dwarfs and neutron stars are then discussed."
Jean Schneider, "A model for a non-chemical form of life: Crystalline physiology," Origins of Life, Volume 8, pages 33-38, 1977.