Abstract
This thesis analyses the concept of biological monstrosity. This concept questions the nature of life and the power of reason. We study the teratology of E. and I. Geoffroy Saint-Hilaire in order to understand how they are successful to think monsters rationally. But both forget the normative dimension in the concept of monster. We try to take this normative dimension into consideration. What should be life to product monstrous beings ? This question leads to develop a metaphysic of life which puts forward the notion of “vital wandering” (“errance vitale”) and the notion of “variation” (“écart”)