Knowledge and Action according to Descartes

Perception caused by action of external things on my body.

"Having sensation is properly something merely passive, just like the shape
that wax gets from a seal."

"When the external sense (organ) is disturbed by the object, the image it
receives is transmitted to another part of the body, called the organ of common
sensibility."

Bodily action is caused by mental acts

"The power of movement... originates in the brain...."

Knowing is a mental activity

"The power of cognition ... is purely spiritual.... Sometimes it receives
images from the common sensibility ...sometimes it applies itself to the images
preserved in memory; sometimes it forms new images.... The cognitive power is
always one and the same...."

(Rules for the Direction of the Mind, before 1630)

My body as a natural thing works according to laws
But in so doing it does not always give me knowledge

This physiology a promissory note
     Descartes doesn't have any good evidence to support a mechanistic view of the body
     A priori philosophical argumentation surely doesn't show it to be plausible

Other views would be consistent with his earlier argument

But it was a way of thinking which has been very productive.

Descartes' Dualism.

The world is a causal system
     It follows purely mechanical laws
But we act freely
     our actions are freely chosen, i.e. not caused
our minds are outside that system

There can be no laws describing our mental activities
     The acts of the soul are free.

The Mind/Body connection

"The various disturbances that the nerves... produce in the brain have various
effects on the mind or soul, which is intimately united to the brain."

"The soul has sensations... only by its presence in the brain."

"The objective external realities that we designate by the words light, color,
odor, flavor or sound, or by names of tactile quantities such as head and
cold... are... nothing other than the powers that objects have to set our
nerves in motion."

The body is a machine (with very small parts):

"The only difference I can see between machines and natural objects is that the
workings of machines are mostly carried out by apparatus large enough to be
readily perceptible by the senses... whereas natural processes almost always
depend on parts so small that they utterly elude our senses...."

But note Descartes' cautious conclusion

"I submit everything to the authority of the Catholic Church, and to the
judgment of wiser heads...."

(Principles of Philosophy, 1644)