it has a certain determinate quantity; and this, we easily understand, may be constant in the universe as a whole while varying in any given part. Thus if one part of matter moves twice as fast as another which is twice as large, we must consider that there is the same quantity of motion in each part; and if one part slows down, we must suppose that some other part of equal size speeds up by the same amount. For we understand that God’s perfection involves not only his being immutable in himself, but also his operating in a manner that is always utterly constant and immutable. Now there are some changes whose occurrence is guaranteed either by our own plain experience or by divine revelation, and either our perception or our faith shows us that these take place without any change in the creator; but apart from these we should not suppose that any other changes occur in God’s works, in case this suggests some inconstancy in God. Thus, God imparted various motions to the parts of matter when he first created them, and he now preserves all this matter in the same way, and by the same process by which he originally created it; and it follows from what we have said that this fact alone makes it most reasonable to think that God likewise always preserves the same quantity of motion in matter …
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.
I will not here add anything about shapes or about the countless different kinds of motions that can be derived from the infinite variety of different shapes. These matters will be quite clear in themselves when the time comes for me to deal with them. I am assuming that my readers know the basic elements of geometry already, or have sufficient mental aptitude to understand mathematical demonstrations. For I freely acknowledge that I recognize no matter in corporeal things apart from that which the geometers call quantity, and take as the object of their demonstrations, i.e. that to which every kind of division, shape and motion is applicable. Moreover, my consideration of such matter involves absolutely nothing apart from these divisions, shapes and motions; and even with regard to these, I will admit as true only what has been deduced from indubitable common notions so evidently that it is fit to be considered as a mathematical demonstration.
Specimen Questions
1 Explain Descartes’s theory of matter as ‘extended substance’. Why was it suited to the ‘new’ quantitative approach to science?
2 Why is extension the principal attribute of matter or physical substance in Descartes’s view? Why are hardness, weight or colour not an essential part of bodily nature?
3 What, on Descartes’s account, individuates the objects of the world?
Suggestions for Further Reading (Including Internet Resources)
1 See readings on Descartes at the end of Part I, extract 4.
2 For a stimulating discussion of the Cartesian revolution in physics and metaphysics, see D. Garber, Descartes’ Metaphysical Physics (Chicago: University of Chicago Press, 1992).
3 For a valuable collection of scholarly essays, see J. Broughton and J. Carriero, A Companion to Descartes (Oxford: Blackwell, 2008).
4 There are free online resources available. The Stanford Encyclopedia of Philosophy has excellent entries on Descartes, his life and times and his philosophy, including links to major works. See https://plato.stanford.edu/entries/descartes/ (by G. Hatfield), and H. Robinson for an illuminating entry on the history of philosophical debates on substance at https://plato.stanford.edu/entries/substance/. Further useful entries can be found in the Internet Encyclopedia of Philosophy, amongst them https://www.iep.utm.edu/descarte/ (by J. Skirry).
5 Among the podcasts made available by the Open Educational Resources program at Oxford University you can find a number of General Philosophy lectures from 2010 including those in which P. Millican explains Descartes’s account of substance at https://podcasts.ox.ac.uk/43-cartesian-dualism and modern responses to Cartesian dualism at http://podcasts.ox.ac.uk/44-mind-body-problem.
Notes
* René Descartes, Principles of Philosophy [Principia Philosophiae, 1644], Part I, articles 51, 52, 54, 63; Part II, articles 1, 2, 3, 4, 21, 22, 23, 36, 64. Trans. J. Cottingham, R. Stoothoff and D. Murdoch, in The Philosophical Writings of Descartes, vol. I (Cambridge: Cambridge University Press, 1985), pp. 210–11, 215, 223–5, 232, 240, 247.
1 1 Thomas Aquinas, Sententia super Metaphysicam [1269–72], IV, 1, 540–3.
2 2 Cartesian: belonging to Descartes (from ‘Cartesius’, the Latin version of his name).
3 3 Concurrence: the continuous conserving power of God necessary to keep things in existence.
4 4 Compare the doubts raised in the First Meditation: see above, Part I, extract 4.
5 5 For more on Descartes’s view of the human being as combination of incorporeal mind and extended body, see below, Part IV, section 4.
6 6 Descartes here rejects the traditional doctrine of a radical difference between ‘sublunary’ or terrestrial phenomena and the incorruptible world of the heavens.
4 Qualities and Ideas: John Locke, Essay Concerning Human Understanding*
A consequence of the new ‘mathematicized’ physics of Descartes was that a certain gap opened up between the quantitative description of the world put forward by the scientist and the ‘common sense’ world revealed by the five senses – the world of colours, smells, tastes, sounds and textures. Descartes had pointed out that nothing reaches the brain from the outside world except various ‘local motions’ transmitted via the sense organs; and he concluded that ‘the properties in external objects to which we apply the terms “light”, “colour”, “smell”, “taste”, “sound”, “heat” and “cold”… are so far as we can see simply various dispositions in the shapes, sizes, positions and movements of their parts that make them able to set up various kinds of motions in our nerves, which then produce all the various sensations in our soul’.1 Taking up this theme, the English philosopher John Locke made a radical distinction between primary and secondary qualities of things. Primary qualities such as shape, he argues in the following extract from the Essay concerning Human Understanding (which appeared at the end of 16892), are ‘utterly inseparable from the Body in whatsoever state it be’. Descartes, as we have seen, took being extended in three dimensions as the essential characteristic of matter; Locke’s list of the basic or primary qualities of matter comprises ‘solidity, extension, figure [i.e. shape] and mobility’.
Now as far as our ordinary idea of the objects around us is concerned, we normally conceive of them as having many other qualities in addition to those on Locke’s list – the marigold has a striking colour, the pineapple a characteristic taste, the perfume a distinctive aroma, and so on. But such qualities are, for Locke, merely powers which objects have to produce various sensations in us by means of their primary qualities (here Locke again owes much to his illustrious French predecessor: compare the sentence from Descartes quoted in the previous paragraph). The upshot is that when we
