1/10. Descartes Laws of Nature

Transcription

1 1/10 Descartes Laws of Nature Having traced some of the essential elements of his view of knowledge in the first part of the Principles of Philosophy Descartes turns, in the second part, to a discussion of the principles of material objects. Effectively then, he turns from metaphysics proper to a form of physics. What we will be concerned with however is the reasons for seeing his account of bodies to be grounded on a set of metaphysical principles. In this way we will confirm the view that Descartes physics is itself metaphysical. This does not however mean that we will be arguing that it is primarily due to his appeal to principles about the properties of God that will be basic to his account although we will see that these appeals do have an important place in Descartes view. The nature of the place they have is however something that will affect different kinds of account of the metaphysical basis of Descartes view of bodies. The first point to remember concerning Descartes view of bodies, and one of the first points he stresses in the second part of the Principles is that he views the nature of body to consist essentially in extension. The argument presented in paragraph four for this view is that it is possible to subtract all other properties from bodies other than extension and still have the nature of body. Least this seem like a poor argument that merely assumes what needs to be proved it is worth pointing out that Descartes gives an example here of his point. Whilst bodies are often hard in the sense Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

2 2/10 that they manifest some resistance to us it is conceivable that instead of manifesting this resistance they could instead simply withdraw from us whenever we moved towards them and in other respects still have the basic nature of body, something which shows that this cannot reside in the possession of the property of hardness. It is similarly possible however to conceive of a body without the various other properties we take it normally to have without it following that we are not dealing with matter at all but if extension was taken away from the body all the other properties would vanish as well which shows that it is extension that is the principal property of body. The next important point that Descartes makes after the argument for taking the essential property of body to reside in extension concerns the account of the relationship between a body s volume (or, as he puts it, internal space ) and its location with respect to other bodies (or, as he terms it, external space ). It follows from his identification of the principal property of body as extension that there is no difference in principle between the body and its internal space. The way we conceive of the difference is that we consider the extension of the body as something individual or specific to the body whilst we think of space as something generic thereby treating space as something that can be filled by things that have different extensions. This difference is a logical one or, following the distinctions of the first part of the Principles, a rational distinction not a real Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

3 3/10 one. A further argument is given in paragraph 11 for this consideration which again supports his claim that extension is the principal property of body. Again, Descartes proceeds by means of subtraction, taking away hardness due to the fact that bodies can be melted or smashed into powder so that they become soft but are still bodies. Colour can also be removed due to the fact that bodies can be transparent and weight as fire has no weight but is clearly still material. Extension is what remains. Having identified body with internal place Descartes next turns to considering external place by means of which we determine the situation of one body in relation to others. However, as he points out in paragraph 13, this introduces the question of frameworks of reference as, to fix a sense of the situation of one body we need to consider other bodies as being at rest relative to it. When a ship moves out to sea a person seated on it stays still relative to the other parts of the ship as he is in the same situation with regard to them but the same person is changing place relative to the shores as he is moving away from some and towards others. Descartes assumes, on the basis of his identification of body with extension that there cannot be a vacuum, that is, a space in which there is contained no matter. Since any space must be considered in relation to extension there must always be some substance contained in it as far as he is concerned. The argument for vacuum is considered by him as based on attending only to what is evidently perceptible and argues that in the case Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

4 4/10 when we can perceive no body (for example in a glass we take to be empty) there must be a body there which we are not perceiving as otherwise the parts of the body would all touch each other. Further, this account of matter renders all bodies homogeneous in nature regardless of the material elements of them. In other words, there is no more matter in a vessel that is filled with lead than one that is full only of air as the latter is as much of a body as the former. It follows also from Descartes view that there are no atoms in the classical sense of indivisible bodies as any infinitesimal bodies must be further divisible given that they are all by their nature extended and hence the world is itself an indefinite whole. The essential elements of body have thus been identified in terms of extension and divisibility. To this Descartes now adds the property of movement to body and his characterization of this will lead to his formulation of three laws of nature. The addition of the property of motion is declared, in the title of paragraph 23, to be necessary in order to grasp the variety of matter or, in other words, diversification emerges as a result of motion. Descartes distinguishes between two senses of motion mentioning both what he calls the ordinary or vulgar sense given to it and the proper sense of it. Before we look at the difference however it is worth mentioning that Descartes states that by movement he means, even in the ordinary sense, only local movement. Local movement is one of the four senses of motion according to Aristotle and meant, to him, change from Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

5 5/10 place to place rather than generation or corruption (change of substance), growth or decay (change of quantity) or alteration (change of quality). So, by only understanding movement as local Descartes distinguishes treatment of it from a general discussion of change. Since Descartes includes this view of local motion in his description of the ordinary view of motion it is clear that it is not the restriction of motion to this characterisation that he objects to in the notion of movement that is there described. The ordinary view of motion consists in the view that it consists in the action by which some body travels from one place to another but this appears no more than a description of local motion. This is followed by a restatement of the point that, on this criterion, assessing what is moving and what is not, appears very difficult since we seem to have view motion in only relativist terms. Further, staying with the example of the person on the ship, it would appear that this person cannot be said to be truly moving since, on the description of motion in terms of action, this person is more obviously at rest than moving as he feels no action in his body. So not only does there appear here to be an intrinsic relativism to this conception but, in its introduction of a notion of action, it appears to produce a view of motion that prevents us from saying that something is moving if it is not itself acting. Paragraph 25 subsequently gives Descartes account of what movement properly speaking consists in. The definition given here is that Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

6 6/10 movement is: the transference of one part of matter or of one body, from the vicinity of those bodies immediately contiguous to it and considered as at rest into the vicinity of [some] others. This view of motion does not immediately seem that different from the ordinary view of the previous paragraph whose problems we have just clarified. However, there are some differences that can be pointed out if we look at the two formulations carefully. The first point that is worth noting is that the proper view contains no reference to action. The second point is that place has now been replaced by vicinity. The third point is that the motion is related to the bodies in its vicinity in terms of these other bodies being considered as at rest. The reason why action is missing from the proper view of movement is evidently to prevent the emergence of the absurd consequence that the person on the boat is not taken to move due to their motion not being a product of their own action. To this point is added a further one when Descartes argues next that there is no more action required to bring about movement than to bring about rest. To bring things to rest requires action of one body on another and this action may be no less than that required to initiate movement. The replacement of place with some other term is clearly needed due to the way in which place leads us to a clearly relativist view of motion but this is not just a replacement of one word by another as the discussion of vicinity is closely connected to the basis of Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

7 7/10 consideration of some bodies as being at rest. This consideration relates to giving an account of the nature absolutely belongs to movement alone, without reference to any other thing as Descartes explicitly puts it in paragraph 29. When we do this we see two bodies moving away from each other and we should say that they are both as much in movement as each other. Now these two bodies are not only moving relative to other bodies but they are also moving on the surface of a body, namely, the body of the earth so if we declare them both to be moving, when both are in fact parts of the general body that is the earth what is to prevent us from saying that the earth is itself moving in two divergent directions? In responding to this question Descartes states that we do not consider a body to be moving unless the whole of it moves so that we cannot think the earth is in motion simply because parts of it are moving from one vicinity to another. This is not, however, his main point since the relative reference frame for the assessment of movement is provided if we restrict our attention to bodies of equivalent size. So, in a sense, what Descartes has done is not to argue against relativist views of motion per se as to show the problem with relativism within frames of reference in order to be able to show the point of the distinctions within them between movement and rest. The point of the distinction within a given framework is precisely to account for reasons why it is right within them to speak of certain things as moving whilst leaving it open to shift frames of reference Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

8 8/10 in order, in shifting them, to describe a framework which enables a different accounting of movement that does not however change the nature of what has been described. This point is clarified when Descartes subsequently argues that each body has only one movement that is peculiar to it since it is understood to move away only from a certain number of bodies that are contiguous to it and considered as at rest. A watch being worn by a sailor whilst traveling on board a ship participates in the movements of the sailor, the ship and the earth itself but it will suffice for us to consider in each body the one movement which is peculiar to it (II 31). So in the proper view of movement we adopt a method of isolation by which the movement internal to each body is defined and described distinctly from the movements of the things to which it is related. It is possible to decompose movements into their elements in order to understand the parts of the movement in terms of an assessment of overall direction but this again does not detract from the emphasis on the one movement proper to each body. Having considered movement in itself we now need to connect it to causality. Given the account of matter s principal attribute as extension it is not evident why bodies should ever begin to move and in paragraph 36 Descartes has to appeal to something beyond extension to explain this beginning of movement by arguing that God is the prime cause of motion. Not only is God the cause of motion but the quantity of it that is present in Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

9 9/10 the universe is conserved by God s action so that its total amount never alters despite its greater or less presence in individual parts. This general metaphysical principle of conservation is the basis of the three laws of nature that follow. The first law states that each thing remains in the same state as far as it is possible for it to do so with it following from this that when it begins to move it will continue to do so. This follows the logic of the general conservation principle since just as motion in matter in general requires reference to a principle external to matter, so the change of state of any individual body must be a consequence of the action upon it of some other body. As Descartes directly puts it, if a body is at rest it will not begin to move unless made to do so by some external cause. It also follows from this that bodies do not have a natural inclination to rest but merely to remain constant in whatever state (motion or rest) they are currently in. The second law of nature tells us that movement is naturally linear. Descartes formulation of the second law refers to movement of itself which indicates that although we are apparently articulating a law in which causes of motion are being adduced that the cause in question here is of a piece with the general characterization of movement and does not yet require direct consideration of physical force. However this law introduces a reference to time since God s sustaining action is said in relation to the determination of matter in motion to require maintenance precisely as it is at the very moment at which He is maintaining it (II 39). The example here Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University

10 10/10 concerns the movement of a stone in a sling. It has a tendency to continue in the determinate direction of linearity in terms of its own motion and is only impelled in a different direction by a force external to its own proper movement. It is only with the third law that we arrive at Descartes account of interaction and we will view this next time in relation to Spinoza s response to Descartes. Gary Banham & Manchester Metropolitan University, 2008 Department of Politics and Philosophy, Manchester Metropolitan University