For its elliptical style, What is Philosophy? appears to be fragmentary and inscrutable, and its reception has been correspondingly contentious. Following an intimation by Gilles Deleuze himself, this article proposes that his final book, written in collaboration with Félix Guattari, contains a philosophy of nature. To address this proposition, the article begins by outlining the comprehensive system of nature set out in What is Philosophy?, defining it as an open system in motion that conjoins philosophy with the historical preconditions and intersects it with science and art. The article then addresses the precise method whereby the philosopher as an individual subject, emerging from nature, can succeed in becoming creative – that is, in creating concepts to bring forth new events. Finally, the brain turns out to be the pivot between the system and this method. What is Philosophy? thus presents an account of the brain based on a theory of the three specific planes of philosophy, science and art, and uses it to expand upon the idea of assemblage for a philosophy of nature.