[Logic and analogy in psychiatry. Their "common root"].

Logic and analogy pose a fundamental problem in psychiatry. The latter is at the moment renewed by the existence of fuzzy logics. The authors put forward the hypothesis of a common root using the systemal method, an original strategy and a particular test. Several questions are tackled: definitions, hierarchy, linking, linearity or circularity, closed of open aspect of thought process, integration, separation or continuity, ways of appearance, discrete organization or organization in networks, different or common origin, importance of modal and non monotonous logics. A discussion and the consequences of this hypothesis are specified.