CLXXXIX explained

What happens

Themes in this chapter

• Mind-Body Dualism

  The chapter describes the soul as united to the whole body but having its principal seat in the brain, where it perceives via nerve motions, illustrating the interaction between mind and body.

Characters to notice

• Narrator

  Explains the nature of perception and the mechanism of sensation.

• René Descartes

  The philosophical author whose views on perception and the soul are presented.

Key passages

• “What perception ( sensus ) is, and how we perceive. We must know, therefore, that although the human soul is united to the whole body, it has, nevertheless, its principal seat in the brain, where alone it not only understands and imagines, but also perceives; and this by the medium of the nerves, which are extended like threads from the brain to all the other members, with which they are so connected that we can hardly touch any one of them without moving the extremities of some of the nerves spread over it; and this motion passes to the other extremities of those nerves which are collected in the brain round the seat of the soul, 36 as I have already explained with sufficient minuteness in the fourth chapter of the Dioptrics. But the movements which are thus excited in the brain by the nerves variously affect the soul or mind, which is intimately conjoined with the brain, according to the diversity of the motions themselves. And the diverse affections of the mind or thoughts that immediately arise from these motions, are called perceptions of the senses ( sensuum perceptiones ), or, as we commonly speak, sensations ( sensus ).”

  Perception is defined as the mind's awareness of sensory input, mediated by nerve motions from the body to the brain, where the soul resides and generates sensations.

  This passage summarizes Descartes' account of sensory perception as a psychophysical process.