Abstract

The aim of this thesis is to defend a reading of Descartes’ theory of sensory perception in which, against a widespread interpretation, the mind is not a passive receiver of inputs from the environment, but an active decoder of neural information that contributes to the representational content of ideas. I call this the ‘mental activity thesis’ and, in the overall picture, I identify it as one of the philosophical implications of the seventeenth-century scientific revolution. Within Descartes’ dualism, to offer a theory of sensory perception amounts to describing the interplay between the natural world, the brain, and the mind. Given his mechanistic, micro-corpuscular conception of matter, Descartes developed detailed physiological descriptions of the interaction between external objects and the brain. He envisaged it as an isomorphic relation in which the characteristics of objects are transmitted through the nerves to the brain as patterns of geometrically reduced properties. This process is often read as culminating with the mind being passively affected by a corporeal isomorph. Descartes’ doctrine becomes elusive in its mental phase, but the passivity reading, so I contend, remains inadequate. I argue for the mental activity thesis through four claims. First, I subscribe the known view that Descartes is concerned about a version of the mind-body problem that is not equivalent to the problem of substance interaction. It is rather a problem of dissimilarity between mental representations and mechanistic explanations. The question is how the qualitative character of sensory experiences can arise from the quantitative notions of physical science. As a way of emphasising the weight that the problem of dissimilarity has for Descartes’ philosophical decisions, I show that it motivates a metaphysically interesting distinction between types of causes for the case of brain-mind interaction. Second, I defend the position that, despite not holding a perfectly unambiguous doctrine, Descartes’ introduction of natural signs is the closest that he got to formulating a full-fledged theory of sensory perception. The appeal to natural signs has been normally deemed as metaphorical in the literature. I argue that, on the contrary, it is possible to reconstruct a causal story for brain-mind interaction along the lines of a semantic model based on Descartes’ identification of neural events with natural signs. A causal-semantic model emerges as a charitable, plausible reading that reveals the mind as an active interpreter. Third, in light of the mental activity thesis, I read Descartes’ late appeal to the innateness of all ideas as a strategy to account for a type of representational content needed for sensory ideas that, while produced by the mind, is different from that of his paradigmatic innate ideas. I assist Descartes in exploring how the category of innateness captures mental activity within a causal-semantic theory. Fourth, in the course of this argumentation, and for further support, I address the role of the distinction between primary and secondary qualities in Descartes’ theory. I tackle a pervasive objection stemming from his alleged association of the perception of primary qualities with the intellect. By reassessing Descartes’ views on mental activity, this interpretation aims at a lucid description of sensory perception that goes beyond the rigid rationalism that is often credited to him.