Cartesian Physics

Descartes' physical theory is fascinating because it attempts to define from the outset any and all phenomena which can be physical under the most simple assumptions, which are three types of matter defined by motion and three laws of motion operating in infinitely divisible space. Descartes' physics in this way is metaphysical. Essentially his physical system amounts to classical mechanics operating in a plenum (space without void) which is itself composed of matter, defined solely by spatial properties, in which if anything changes its state of motion so does anything near it. Bodies with different masses form from local aggregations in space sharing the same state of motion. Massive bodies alter the straight line motion of less massive bodies which resist homogeneity of motion with the large bodies so that they circle around in a vortex of centripetal force, being pulled in with centrifugal motion and by inertia moving in a straight line. 

The method

1. accepting only information you know to be true
2. breaking down these truths into smaller units
3. solving the simple problems first
4. making complete lists of further problems

Descartes famous method of doubt philosophically established only two things or substances that can be certain to exist, matter and mind. Matter is defined solely by extension in space which contains the only qualities of matter which aren't mental. Mind is not extended in space and so cannot be broken down into smaller parts which function the same as the whole. However Descartes did believe mind and body interacted in an intimate fashion, the point of contact being the pineal gland in the brain. Though mental qualities are qualitatively different than material qualities, the physical correlates of mental activity, the passions, operate by the same rules as all matter.

"The same extension in length, breadth, and depth, which constitutes space, constitutes body; and the difference between then lies only in this, that in body we consider extension as particular...whereas in space we attribute to extension a generic unity". space is internal place, a mode in which they are conceived.

The next key principle of Descartes' physical theory was a denial of the void. The Ancient Greek atomists believed that very small uncuttable particles composed everything, comparing apart and coming together in an empty void. The pre-Socratic philosophers Parmenides and Zeno denied the void for how could there be nothing? Existence means something instead of nothing, and so the void would be something, otherwise would be contradictory. This was very important to them since if there is not nothing, then change is impossible. For how could one thing be at one place at one time and then another place at another moment without having to go through an infinite number of steps to get there (Zeno's paradox)? Instead, matter is infinitely divisible. There are no uncuttable atoms. This is important to Descartes because he defines matter solely by space, and so all material things have the same properties which fall under the simple and certain definition of extension. It also means material bodies have no essential organizing principle to their order, which fits with Descartes' mind-body dualism. This allows mechanistic explanations to overcome scholastic physics. 

"With regard to vacuum, in the philosophical sense of that term, that is, a space where there is no substance, it is evident that such does not exist, seeing the extension of space or internal place is not different from that of body...since there is extension in it there is necessarily also substance."

Vacuum: "a place in which there is none of those things we presume ought to be there"

On atoms: "for however small we suppose these parts to be, yet because they are necessarily extended, we are always able in thought to divide any one of them into two or more smaller parts...he [God] could not deprive himself of the ability to do so"

"We still not only imagine beyond it spaced indefinitely extended, but perceive these to be truly imaginable"

Using his method, Descartes thinks that he can explain the workings of the entire material world. 

"There is no phenomenon of nature whose explanation has been omitted in this treatise; for beyond what is perceived by the senses, there is nothing that can be considered a phenomenon of nature"

The Three Types of Matter, which differ by quality of motion

First element- "parts move so quickly and are so small that no other body is able to stop them and, in addition, they do not require any determinate size, shape, or position." Sources of light, the sun and the stars.

Second element- "it's parts have a motion and size that are so moderate that, if there are many causes in the world which can increase their motion and decrease their size, there must be as many others which can do the exact opposite. Thus they remain permanently balanced in this same moderate condition" Atoms and molecules, ordinary matter for us planet dwellers.

Third element- "it's parts are so large, and so joined together, that they always have the force to resist the motion of other bodies" the earth, planets, and comets.

If something has force to move something else, it decreases its own movement. If its parts move too fast it will break apart and lose its size. There is a tendency for each element to revert to itself, and their mixture is mostly superficial.

Of Motion

"all variation in it, or diversity of form, depends on motion"

In Descartes' physics local motion has to explain a lot. Persistence and conservation explain everything metaphysically

Motion defined: "nothing more than the action by which a body passes from one place to another...the same thing may be said to change and not to change place at the same time, so we may also say that the same thing is at the same time moved and not moved"

Motion proper- "the transporting of one part of matter or of one body from the vicinity of those bodies that are in immediate contact with it, or which we regard as at rest, to the vicinity of other bodies"

"Motion is always in the movable thing, not in that which moves" [reductionism] "it is a mode of the movable thing, and not a substance."

Laws of motion (from Principles of Philosophy)

1) "each thing, in so far as it is simple and undivided, always remains in the same state, as far as it can, and never changes except as a result of external causes...hence we must conclude that what is in motion always, in so far as it can, continues to move" Law of inertia

2) "every piece of matter, considered in itself, always tends to continue moving, not in any oblique path but only in a straight line"

3) "when a moving body collides with another, if it's power of continuing in a straight line is less than the resistance of the other body, it is deflected so that, while the quantity of motion is retained, the direction is altered; but if its power of continuing is greater than the resistance of the other body, it carried that body along with it, and loses a quantity of motion equal to that which it imparts to the other body"

• Explanation: The larger a body is, the greater the persistence in motion or in rest, and the greater its resistance. The faster a body moves, the greater is its persistence in motion, and the greater its resistance to another body. 

• Quantity of Motion: if a body is in motion, its persistence in that state is measured by its size and velocity. 

• Quantity of Rest: "a body at rest gives more resistance to a larger velocity than to a smaller one in proportion to the excess of the one velocity to the other". a body at rest can adjust its resistance to any exterior velocity in a body of equal size, its persistence in rest is reduced to a function of its size. Measured by its size and the velocity of something else of equal size 

The three divisions of matter along with the three laws of motion are responsible for all physical phenomena in Descartes' system, including gravity.

Explanation of gravity: "I know none of them who did not suppose that there was gravity in terrestrial bodies; but although experience shows us very clearly that bodies we call heavy descend towards the center of the earth, we do not, therefore, know the nature of gravity, that is, the cause or principle in virtue of which bodies descend, and we must derive our knowledge our knowledge of it from some other source"- preface

"he [Democritus] attributed gravity to these bodies, of which I deny the existence in any body, in so far as a body is considered in itself, because it is a quality that depends on the relations of situation and motion, which several bodies bear to each other.

Descartes has a relational theory of gravity, not a force in itself. This is in contrast to Issac Newton's theory of gravity as a universal attractive force between all bodies with mass, the force inversely proportional to the square of their distances the masses. However Newtonian gravity remained an "occult quality", unexplicable in mechanical terms even though it is true. 

The Vortex
"It has been shown…that all places are full of bodies…. From this it follows that no body can move except in a complete circle of matter or ring of bodies which all move at the same time.” This follows from law of motion 2 that unimpeded motion occurs in a straight line, in application to law of motion 3 which alters direction into circular movement. When one body moves to another space, the previously occupied space is taken by another body and so on depending on the quantity displacement.

Descartes' vortex theory is very similar to the nebular hypothesis for the formation of planetary bodies. Matter builds up in an accretion disk which becomes massive enough that internal and external forces act to create a spherical body.

Time: from meditations. "It is as a matter of fact perfectly clear and evident to all those who consider with attention the nature of time, that in order to be conserved in each moment in which it endures, a substance has need of the same power and action as would be necessary to produce and create it anew, supposing it did not yet exist, so that the light of nature shows us clearly that the distinction between creation and conservation is solely a distinction of the reason.