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Aristotelian
bodies composed of form & matter
form is, or determines, essence
a body’s form directs its development and activity towards an end
sensory perceptions resemble forms
PS: the heavens are immutable (different physics)
Descartes
bodies composed of matter only
bodies have a few simple properties (e.g. extension, motion)
these properties explain all their sensory effects (e.g. heat, colour, taste)
these properties are measurable on ratio scales
Letter to Mersenne (28 January 1641)
‘these six meditations contain all the foundations of my physics. But please do not tell people, for that might make it harder for supporters of Aristotle to approve them. I hope that readers will gradually get used to my principles, and recognize their truth, before they notice that they destroy the principles of Aristotle.’
‘The whole of Philosophy is like a tree, of which the roots are Metaphysics, the trunk is Physics, and the branches which come out of this trunk are all the other sciences ...’
Principles of Philosophy
How to make sense of the metaphors?
What principles did Descartes think physics rested on?
‘The only principles which I accept or require in physics are those of geometry and pure mathematics;
these principles explain all natural phenomena, and enable us to provide quite certain demonstrations regarding them’
Principles
How to make sense of the metaphors?
1. It’s the intellect, not the senses, that enables us to know the essential nature of material bodies.
2. The intellect does not enable us to know forms.
3. The intellect does enable us to know mathematical principles.
4. Those principles must characterise the essential nature of bodies.
External bodies
‘may not exist in a way that
exactly corresponds with my sensory grasp of them,
for in many cases the
grasp of the senses
is very obscure and confused.
But at least they possess all the properties which I clearly and distinctly understand,
that is all those which, viewed in general terms, are comprised within the subject matter of pure mathematics.’
How to make sense of the metaphors?
1. It’s the intellect, not the senses, that enables us to know the essential nature of material bodies.
2. The intellect does not enable us to know forms.
3. The intellect does enable us to know mathematical principles.
4. Those principles must characterise the essential nature of bodies.
‘in the whole history of physics up to now people have only tried to imagine some causes to explain the phenomena of nature, with virtually no success.
Compare my assumptions with the assumptions of others.
Compare all their real qualities, their substantial forms, their elements and countless other such things
with
my single assumption that
all bodies are composed of parts’
Descartes, Letter to Morin, 13 July 1638 (AT 2:196)
Descartes
How could we know the essential nature of bodies?
Through the intellect.
The intellect reveals mathematical principles.
Therefore these principles must characterise the essential nature of bodies.
Undescartes
How could we know the essential nature of bodies?
Bodies are made of up tiny cats.
The intellect cannot reveal the tiny cats.
Therefore we cannot know the essential nature of bodies through the intellect.
?
Discoveries about how we know
support claims
about what is.
How can we acquire knowledge about the essential nature of the bodies located outside us?
New answer -> new theory of essential nature
‘The whole of Philosophy is like a tree, of which the roots are Metaphysics, the trunk is Physics, and the branches which come out of this trunk are all the other sciences ...’
Descates (Principles of Philosophy p. 186; AT IX–2: 14)