was trying to understand how this happens. In the 1600s, one idea
was that this type of action was driven by fluid pressure of some
kind, perhaps liquid or air. The fluid is heated by the fire, resulting in
increased pressure that forces it through channels up to the head and
then back again.
Descartes was very interested in trying to understand perception
—how is it we are able to sense the world, and how is it that physical
sensations lead to mental experiences? His Treatise on Man contains
drawings illustrating dissections of the eye and speculations about
the connections between the eye and the brain (Fig. 2.12).
Figure 2.12. Dissection of muscles surrounding the eyeball, from L’Homme de
René Descartes (1664).
It seemed that light is captured optically by the eye and that signals
must somehow be transmitted to the brain (Fig. 2.13). But then what?
How can the physical sensations of light lead to our mental experience
of the world—to visual perception? Four centuries later, this is still an
outstanding question.
It is now appreciated that signaling in the nervous system is
fundamentally electrical in nature—electrical meaning involving the