Given that the senses are supposed to be almost entirely left out of our search for knowledge, for what good then does Descartes think they are intended? Actually, Descartes thinks that they are very, very good at what they are meant to do, which is to provide us with information that enables us to get around in the world. The senses do not belong to the mind (responsible for our intellectual ideas) and they are not supposed to act as the handmaiden to the intellect, providing it with fodder for scientific reasoning. They also do not belong to the body (joined to the mind in some mysterious way to form a human being). Instead they belong to the composite of mind and body (the whole person), and they tell us what is beneficial and harmful to this composite.

Sensation works through a series of nerve channels, connecting various organs to the brain (the bodily seat of the mind). Our eyes, ears, nose, mouth, and skin are affected by tiny bodies in the air, in water, in other objects, and they send nerve signals to the brain, which result in sensations. Obviously the most mysterious part of this picture is how the stimulation of nerves in the bodily brain can possibly result in the production of sensations in the un-bodily mind. Descartes is not able to give an adequate response to this puzzle of mind-body interaction.