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Next: 3.3 Aristotle's Physics Up: 3. Problems in Classical Previous: 3.1 Newtonian Physics

3.2 Descartes and Leibniz

The other most popular approaches were those of Descartes, and those of Leibniz.     Descartes had imagined that the natural world is made out of res extensa, or extended bodies. That is, the ultimate individuals are those whose nature is just to be extended, and to occupy volumes in space.

The trouble with this alternative is that the problem of disposition is further from solution, not nearer. According to Descartes, it is mysterious why the ultimate individuals should have mass or inertia, or should proceed through space at constant velocity (unless disturbed). It is even not clear what is stopping the individuals from penetrating into each other, and hence passing through each other unchanged, as we can easily imagine extended bodies (such as two geometric spheres) doing this. What would stop inter-penetration would be some solidity or filling of the space that has an ability to repel other individuals, but to say this is to say rather more than the individuals are `extended bodies'.  

    Leibniz, on the other hand, had imagined that the world is made of monads, or `simple substances' that are all independent of each other. He has furthermore that all natural changes of the monad come from within, as `an external cause can have no influence upon its inner being.'3.4 But the trouble is that it is now mysterious how the monads, his simple substances, have the disposition to interact with each other at all. Although Leibniz was a strong advocate of the importance of dynamics and forces in physical explanations, in the end he can not explain how these are real features of his ultimate monads, because the monads only appear to interact with each other, and, as just stated, do not really do so.  

  Descartes, Newton and Leibniz all shared the assumption that the ultimate substances had to be purely actual and definite in their inner form. In particular, they could never be internally changed by interactions with other substances. They assumed, as general principle, that the ultimate substances had nothing in them like a disposition for changing or being changed during interactions. It is of course possible to have some account of forces or potentials grafted onto the account of Newtonian corpuscles, and simply assert the `scientific law' that all matter, for example, attracts all other matter in certain ways. However, even Newton realised that that is hardly a satisfying explanation in the long run.

next up previous contents index
Next: 3.3 Aristotle's Physics Up: 3. Problems in Classical Previous: 3.1 Newtonian Physics
Prof Ian Thompson


Author: I.J. Thompson (except as stated)