From physical to biological time.

Time is a primitive (i.e. fundamental) notion, and the various concepts** that have been so far derived from this notion in various scientific domains do not cover all facets of it. Time in mechanics, either classical, quantal or relativistic, is devoid of directionality, the "arrow", i.e. of irreversibility: these physical theories are fundamentally time reversible. Thermodynamics, however, does involve irreversibility, but only as an empirical observation, rather than as a fundamental law of nature (entropy decreasing processes are said to be very improbable, they are not said to be forbidden). In contrast with physics, the arrow of time is of a tremendous importance and effect in biology. For this reason here I will propose a notion of time that--contrary to the claim of several current epistemological schools--is both primitive and oriented. Time flow and irreversibility are indeed at the heart of phenomena of the generation and growth of biological order (in developing organisms), of phenomena of maintenance of organisms in their healthy adult age (it is suggested that the production and coordination of temporal cycles are as important and perhaps more important in understanding this maintenance than the usually emphasized phenomenon of homeostasis), and finally of phenomena of senescence, with their ultimate issue: death. In all these fields, life obeys--does not negate--the thermodynamic law of increase of entropy, as the development of irreversible processes thermodynamics allows us to understand it. Many biologists use the concept of entropy in a somewhat restricted, and sometimes misleading way, namely as a measure of disorder. But the relationship between entropy and disorder is more subtle than mere equivalence. In order to clarify these ideas, a most precise relationship between entropy and order, using the physical concept of phase space, is expounded and illustrated. Application of the results of thermodynamics of irreversible processes to living beings requires a jump in complexity, the wideness of which is acknowledged: possible specific effects of this jump (linked for instance to the high number of hierarchical levels and/or to the role of randomness in the organization of the lower levels) are mentioned.4+ Some current models of aging, using thermodynamical analogies, are examined and discussed . Finally, it is pointed out that the concepts of time so far examined do not include the notion of the "present", which is so obviously at the heart of our psychological, internal and subjective notion of time.(ABSTRACT TRUNCATED AT 400 WORDS)