Many commentators position Merleau-Ponty's philosophy in stark opposition to Descartes and speak of Merleau-Ponty's fundamental discovery of the lived body as a ‘victory over Cartesianism’. While this affirmation is not completely false, some commentators also acknowledge that Descartes is not merely Merleau-Ponty's adversary. Sara Heinämaa, for example, claims that Merleau-Ponty finds in Descartes ‘a fruitful discussion on the mind-body compound’. Indeed, by describing the union of the soul and the body as a third notion irreducible to both extension and thought, one that necessitates its own, distinct way of being conceived, Descartes also provides Merleau-Ponty with some of the tools necessary to overcome Cartesian dualism. Saint Aubert also qualifies Merleau-Ponty's relation to Descartes as ambivalent and shows how Merleau-Ponty finds his own premises in what he calls the ‘Cartesian tremor [le tremblement cartésien]’ (SC, 21, 38). Descartes, then, plays a double role throughout Merleau-Ponty's thinking: he is both the thinker who is criticised for his ‘ontology of the object’ and the one who provides the tools to overcome this same ontology. What we find in Merleau-Ponty then is never a straightforward rejection of Descartes's thinking, but rather a criticism of the Cartesian – i.e. dualistic – premises that are still at work in our thinking today.

Merleau-Ponty's ambivalence toward Descartes's philosophy is explicit, for example, in his criticism of vision as the ‘thought of seeing’ against which he will develop, with the help of Gestalt psychology, his own phenomenology of perception. While Descartes's Dioptrics remains an important text for Merleau-Ponty, we would be wrong to consider that the Cartesian theory of perception is merely a historical curiosity for Merleau-Ponty. Early on, Merleau-Ponty denounces ‘the pseudo-Cartesianism of scientists and psychologists [who] consider perception and its proper objects as “internal” or “mental phenomena”, as functions of certain physiological and mental variables’. Furthermore, it is not as if vision in act was completely absent from Descartes's philosophy. But as with the union of the soul and the body, vision in act is an enigma that doesn't trouble Descartes's philosophy because it belongs to everyday life and not to philosophy. It cannot be conceived; it can only be exercised or practised (see PoP, 176).