The course on nature coincides with the re-working of Merleau-Ponty's breakthrough towards an ontology and therefore plays a primordial role. The appearance of an interrogation of nature is inscribed in the movement of thought that comes after the Phenomenology of Perception. What is at issue is to show that the ontological mode of the perceived object - not the unity of a positive sense but the unity of a style that shows through in filigree in the sensible aspects - has a universal meaning, that the description of the perceived world can give way to a philosophy of perception and therefore to a theory of truth. The analysis of linguistic expression to which the philosophy of perception leads opens out onto a definition of meaning as institution, understood as what inaugurates an open series of expressive appropriations. It is this theory of institution that turns the analysis of the perceived in the direction of a reflection on nature: the perceived is no longer the originary in its difference from the derived but the natural in its difference from the instituted. Nature is the "non-constructed, non-instituted," and thereby, the source of expression: "nature is what has a sense without this sense having been posited by thought." The first part of the course, which consists in a historical overview, must not be considered as a mere introduction. In fact, the problem of nature is brought out into the open by means of the history of Western metaphysics, in which Descartes is the emblematic figure. The problem consists in the duality - at once unsatisfactory and unsurpassable - between two approaches to nature: the one which accentuates its determinability and therefore its transparency to the understanding; the other which emphasizes the irreducible facticity of nature and tends therefore to valorize the view-point of the senses. To conceive nature is to constitute a concept of it that allows us to "take possession" of this duality, that is, to found the duality. The second part of the course attempts to develop this concept of nature by drawing upon the results of contemporary science. Thus a philosophy of nature is sketched that can be summarized in four propositions: 1) the totality is no less real than the parts; 2) there is a reality of the negative and therefore no alternative between being and nothingmess; 3) a natural event is not assigned to a unique spatio-temporal localization; and 4) there is generality only as generativity.