The body is at the heart of Merleau-Ponty's philosophy. The theme is anticipated by the study of animal and human behaviour in The Structure of Behavior (1942) and is central to Phenomenology of Perception (1945), which is our focus. Against Descartes and the usual contemporary view of the body as a biomedical object or as a vehicle of consciousness, the Phenomenology demonstrates that one is one's body. There is no ontological separation between the experiencing “I” and the body as one lives it. Indeed, the lived body is one's intentional opening to the world, through which alone one experiences meaningful things in the first place.

Merleau-Ponty's philosophy of the body is therefore no mere study of a neutral object, but an investigation of one's existence as a philosopher. So the body is a key methodological term, equal in significance to Descartes's cogito. But the cogito would close philosophical problems by having the philosophizing “I” certify itself from within. In contrast, Merleau-Ponty poses philosophy's initial question – “Who am I?” – within a body open to the world. This inherently exposes philosophy to living, perceptual, emotional, sexual and expressive drives, to other people, to lived space and time – it uncovers philosophy's permanent openness to what Merleau-Ponty later calls the pre-philosophical or pre-theoretical (cf. N: 72; S: 164–5; PrP: 3–6). Merleau-Ponty thus radicalizes philosophical method, since his philosophy begins by installing itself in and being responsible to a pre-philosophical setting that exceeds it.