In philosophy of mind, Cartesian materialism is the idea that at some place (or places) in the brain, there is some set of information that directly corresponds to our conscious experience. Contrary to its name, Cartesian materialism is not a view that was held by or formulated by René Descartes, who subscribed rather to a form of substance dualism.

In its simplest version, Cartesian materialism might predict, for example, that there is a specific place in the brain which would be a coherent representation of everything we are consciously experiencing in a given moment: what we're seeing, what we're hearing, what we're smelling, and indeed, everything of which we are consciously aware. In essence, Cartesian materialism claims that, somewhere in our brain, there is a Cartesian theater where a hypothetical observer could somehow find the content of conscious experience moment by moment. In contrast, anything occurring outside of this privileged neural media is nonconscious.

The homunculus argument is an informal fallacy whereby a concept is explained in terms of the concept itself, recursively, without first defining or explaining the original concept. This fallacy arises most commonly in the theory of vision. One may explain human vision by noting that light from the outside world forms an image on the retinas in the eyes and something (or someone) in the brain looks at these images as if they are images on a movie screen (this theory of vision is sometimes termed the theory of the Cartesian theater: it is most associated, nowadays, with the psychologist David Marr). The question arises as to the nature of this internal viewer. The assumption here is that there is a little man or homunculus inside the brain looking at the movie.

The reason why this is a fallacy may be understood by asking how the homunculus sees the internal movie. The answer[citation needed] is that there is another homunculus inside the first homunculus's head or brain looking at this movie. But that raises the question of how this homunculus sees the outside world. To answer that seems to require positing another homunculus inside this second homunculus's head, and so forth. In other words, a situation of infinite regress is created. The problem with the homunculus argument is that it tries to account for a phenomenon in terms of the very phenomenon that it is supposed to explain