The living world is one of complexity, the result of innumerable interactions among organisms, cells, molecules. In analyzing a problem, the biologist is constrained to focus on a fragment of reality, on a piece of the universe which he arbitrarily isolates to define certain of its parameters.In biology, any study thus begins with the choice of a “system.” On this choice depend the experimenter's freedom to maneuver, the nature of the questions he is free to ask, and even, often, the (...) type of answer he can obtain. (shrink)