Philosophy and Cosmology: A Revolution

Transcription

1 Philosophy and Cosmology: A Revolution PHIL Dr Katherine Brading kbrading@nd.edu 316 Malloy Hall All the slides used in class can be found by following the appropriate links at 1

2 5. Cartesian Cosmology The story so far... The old cosmology is giving way to the new cosmology: a Sun-centered universe with the Earth orbitting the Sun Rene Descartes What moves the planets? Kepler: the Sun moves the planets How can we respnd to the arguments against the motion of the Earth? Galileo: -principle of inertia -principle of relativity The old cosmology is crumbling. How can we ensure that our new cosmology is built on a firm foundation? Enter Descartes... 2

3 5.1 A firm foundation Descartes Meditations on First Philosophy, 1641 Aim: To establish in the sciences something stable and likely to last. Method: To hold back assent from opinions which are not completely certain and indubitable. Problem: It would be an endless task to examine each of our beliefs one-by-one. Solution: Address the basic principles on which all our beliefs are based. Problem: What are these basic principles? Solution: The beliefs that we take to be most certain are those which depend on our present sense perceptions. We should therefore address the principle that the senses provide us with beliefs that are certain. 3

4 Interlude: sources of knowledge Recall our discussion from the beginning of the semester: What are our sources of knowledge? 4

5 Interlude: sources of knowledge Recall our discussion from the beginning of the semester: What are our sources of knowledge? authority sense perception anything else? What is our most reliable form of knowledge? Knowledge based on sense perception? This is where Descartes starts when he is trying to find a secure foundation for scientific knowledge. Let s take a look at how he goes about it. 5

6 Descartes, Meditation I (1) What is the argument from perceptual error? What does this argument call into doubt? (2) What is the argument from dreaming? What does this argument call into doubt? (3) What is the argument using the evil genius? What does this argument call into doubt? (4) By the end of Meditation I, what has the method of doubt shown us about the status of our present sense perceptions? 6

7 Descartes, Meditation II I think, therefore I am (stated this way in the Discourse on Method) Here, at last, is something which is indubitable. 7

8 Descartes, Meditation III From Med II: I think, therefore I am is indubitable. From this one case of certainty, can I establish general criteria for certainty? Suggestion: I know that I am a thinking thing because I have a clear and distinct perception that I am a thinking thing therefore all the things that I perceive clearly and distinctly are true. Problem: This will only work if there is a non-deceiving God who guarantees that my clear and distinct perceptions do not lead me astray. 8

9 Clear and distinct ideas and intelligibility According to the proponents of the new mechanical philosophy (we ll come back to this in more detail in Topic 6): The Aristotelian cosmos is mysterious and unintelligible The new cosmology must be intelligible: everything in our new cosmology should be clear and distinct to our intellect. 9

10 Overview of Topic A firm foundation (The Meditations, 1641) 5.2 Mind and matter (The Meditations, 1641) 5.3 Descartes s intelligible cosmos 5.4 Motion 5.5 The cosmology of The World (c. 1629) 5.6 A new theory of motion 5.7 Descartes s laws of nature 5.8 The cosmology of the Principles of Philosophy (1644) 5.9 Review 10

11 Descartes s Meditations: 1. Of the things we may doubt - evil genius 5.2 Mind and matter 2. Of the Nature of the Human Mind; and that it is Easier to Know than the Body - my existence as a thinking thing - the wax example 3. Of God; That He Exists - a general criterion? - the connection between clear and distinct ideas and truth is guaranteed by a non-deceiving God 4. Of Truth and Error - if there is a non-deceiving God, how come I make mistakes? 5. Of the Essence of Material Things; and, Once More of God, that He Exists 6. Of the Existence of Material Things, and of the Real Distinction between the Soul an the Body of Man 11

12 Upshot of the Meditations: Cartesian Dualism There are two substances in the world mind (essential property: thinking) body (essential property: being extended) Each is capable of existing without the other. Crucial for our purposes: the sole essential property of matter is extension 12

13 Cartesian Dualism There are two substances in the world mind (essential property: thinking) body (essential property: being extended) Each is capable of existing without the other. A human being is a soul (= a thinking thing) united to a body (= an extended thing) My soul is capable of existing without a body: it is an independent thing If a soul ceases to think, it ceases to exist 13

14 Cartesian mind-body problem Claim: Pain, hunger, thirst etc teach me that I have a body, and furthermore that I am joined to it very closely and indeed so compounded and intermingled with my body that I form, as it were, a single whole with it. Grounds for this claim: If I were not so joined to my body I would judge my body to be in pain (for example) but I would not feel the pain. This distinguishes my experience of my own body from my experience of other bodies. Questions: Given Cartesian dualism, how do mind and body interact so that I can will my arm to move, and so that I feel sensations of pain, etc? What does intermingling really mean? Does intermingling undermine mind-body dualism? 14

15 5.3 Descartes s intelligible cosmos built on a foundation of clear and distinct ideas All knowledge of the material world is based on clear and distinct ideas Reject earth, air, fire, and water as the basic elements of the universe from which everything material is made Why? What makes our ideas of earth, air, fire and water not clear and distinct enough? Answer: We know their properties through sensation. And sensory experience is fuzzy and indistinct, arising as it does from the intermingling of mind and body. God gave us our senses to help us get around in the world safely, but our senses don t tell us what the world is really like in itself. To know what the world is really like in itself, we have to use our reason. 15

16 Descartes s intelligible cosmos built on a foundation of clear and distinct ideas All knowledge of the material world is based on clear and distinct ideas Reject earth, air, fire, and water as the basic elements of the universe from which everything material is made To know what the world is really like in itself, we have to use our reason. When we consider our ideas of bodies, and we abstract all the indistinct aspects that arise from our sensory experience of bodies, what properties of bodies are left? Answer: extension (essential) and motion (we ll come back to this) the sole essential property of matter is extension the material world consists of matter in motion 16

17 Descartes, The World: Treatise on Light Chapter 1: On the Difference Between our Sensations and the Things That Produce Them In putting forward an account of light, the first thing that I want to draw to your attention is that it is possible for there to be a difference between the sensation we have of it, that is, the idea that we form of it in our imagination through the intermediary of our eyes, and what it is in the objects that produces the sensation in us, that is, what it is in the flame or in the Sun that we term light. For although everyone is commonly convinced that the ideas that we have in our thought are completely like the objects from which they proceed, I know of no compelling argument for this. Quite the contrary, I know of many observations which cast doubt on it. If you find it strange that do not use the qualities called heat, cold, moistness, and dryness, as the Philosophers do, I shall say that these qualities appear to me to be themselves in need of explanation. Indeed, unless I am mistaken, not only these four qualities but all others as well, including even the forms of inanimate bodies, can be explained without the need to suppose anything in their matter other than motion, size, shape, and arrangement of its parts. 17

18 Descartes s intelligible cosmos built on a foundation of clear and distinct ideas All knowledge of the material world is based on clear and distinct ideas Reject earth, air, fire, and water as the basic elements of the universe from which everything material is made Instead, choose the basic constituents of the universe to be those of which we have a clear and distinct idea: the pure extension of geometry, and motion The material world consists of matter in motion The sole essential property of matter is extension The parts of matter (= bodies) move according to three simple laws of nature (we ll come back to these) From these basic principles, everything about the material world can be explained... 18

19 5.4 Motion Descartes s intelligible cosmos built on a foundation of clear and distinct ideas All knowledge of the material world is based on clear and distinct ideas The material world consists of matter in motion The sole essential property of matter is extension The parts of matter (= bodies) move according to three simple laws of nature (we ll come back to these) From these basic principles, everything about the material world can be explained... But wait! A clear and distinct idea of motion? 19

20 But wait! A clear and distinct idea of motion? Imagine you were coming to university in the seventeenth century. What would you learn about motion? 20

21 But wait! A clear and distinct idea of motion? Daniel Sennert ( ), Doctor of Physick Thirteen Books of Natural Philosophy (1660) Book 1, Chapter 9: Concerning Motion We now come to a remarkable Adjunct of Natural Bodies, viz. Motion, by the knowledge of which we are brought acquainted with the greatest part of the most abstruse things in Nature; and by the guidance whereof, in a manner all the knowledge of Natural things hath been found out. 21

22 But wait! A clear and distinct idea of motion? Daniel Sennert, Doctor of Physick Thirteen Books of Natural Philosophy (1660) Book 1, Chapter 9: Concerning Motion We now come to a remarkable Adjunct of Natural Bodies, viz. Motion, by the knowledge of which we are brought acquainted with the greatest part of the most abstruse things in Nature; and by the guidance whereof, in a manner all the knowledge of Natural things hath been found out. Now to the Question whether there is any Motion, is the part only of a contentious and forward person: since it is discerned by our senses. And therefore Diogenes did well, who having heard of the reasons of Zeno, whereby he endeavoured to prove that there was no motion, he judged them not worth the answering, but only Rose up and walked, intimating that in things subject to sense, we should believe our senses. Which Aristotle also approves in his 8. Physik. Chap. 3. T. 32 to say that nothing moves, and to go about to prove it, omitting our senses, is weakness of Understanding. 22

23 But wait! A clear and distinct idea of motion? Daniel Sennert, Doctor of Physick Thirteen Books of Natural Philosophy (1660) Book 1, Chapter 9: Concerning Motion We now come to a remarkable Adjunct of Natural Bodies, viz. Motion, by the knowledge of which we are brought acquainted with the greatest part of the most abstruse things in Nature; and by the guidance whereof, in a manner all the knowledge of Natural things hath been found out. Now to the Question whether there is any Motion, is the part only of a contentious and forward person: since it is discerned by our senses. And therefore Diogenes did well, who having heard of the reasons of Zeno, whereby he endeavoured to prove that there was no motion, he judged them not worth the answering, but only Rose up and walked, intimating that in things subject to sense, we should believe our senses. Which Aristotle also approves in his 8. Physik. Chap. 3. T. 32 to say that nothing moves, and to go about to prove it, omitting our senses, is weakness of Understanding. But as it is most known that there is motion : so what is the Nature thereof is sufficiently obscure. And therefore, that we may better understand what motion is, we will in the first place propound Aristotle s definition of motion, and diligently consider the same : by which means we hope to attain a good degree of knowledge touching the Nature of motion. Thus therefore Aristotle defines Motion in the 3. of his Physicks, Ch 1. T. 16. Motion is the Act of that thing which is in a possibility, in as much as it is in a possibility, or of that which is moveable, in as much as it is Moveable. 23

24 Daniel Sennert, Doctor of Physick Thirteen Books of Natural Philosophy (1660) Book 1, Chapter 9: Concerning Motion Motion is the Act of that thing which is in a possibility, in as much as it is in a possibility, or of that which is moveable, in as much as it is Moveable. There follows several pages of discussion and analysis, at the end of which Sennert concludes: Motion therefore properly is only made out of one subject into another: and there are three sorts thereof. In Quantity there is Augmentation and Diminution. In Quality, there is Alteration. In Place, there is local motion from place to place. To Motion Rest is opposed, which is the Privation of Motion in that thing, which is naturally apt to be moved; and in as much as motion is made to attain rest, it may also be said to be the end and perfection of motion. Rest is either natural, whose principle is internal, and to which the movable is naturally inclined: So the Earth is naturally moved to the Centre, and rests there. Or Violent, whose Principle is external, when a body is kept by force in a place not natural thereunto; as when a stone or clod of Earth is hung up in the Air. A clear and distinct idea of motion? 24

25 Sennert on motion (1660) Motion therefore properly is only made out of one subject into another: and there are three sorts thereof. In Quantity there is Augmentation and Diminution. In Quality, there is Alteration. In Place, there is local motion from place to place. Newton s Laws (Principia, 1687) Law I Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. Law II The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. Law III To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. How did we get from here to there? Descartes played an important role... 25

26 Just to make that vivid, compare Descartes s laws with Newton s: Descartes Laws of Nature (Principles of Philosophy, 1644) 1 Each and every thing, in so far as it can, always continues in the same state; and thus what is once in motion always continues to move. 2 All motion is in itself rectilinear. 3 If a body collides with another body that is stronger than itself, it loses none of its motion; but if it collides with a weaker body, it loses a quantity of motion equal to that which it imparts to the other body. Newton s Principia (1687) Law I Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. Law II The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. Law III To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. 26

27 Descartes, The World, c1630, Chapter 7 [The Philosophers] themselves admit that the nature of their motion is very little understood. And trying to make it more intelligible, they have still not been able to explain it more clearly than in these terms: Motus est actus entis in potentia, prout in potentia est. These terms are so obscure to me that I am compelled to leave them in Latin because I cannot interpret them. (And in fact the words motion is the act of a being which is in potency, in so far as it is in potency are no clearer for being in the vernacular.) By contrast, the nature of the motion that I mean to speak of here is so easily known that even geometers, who among all men are the most concerned to conceive the things they study very distinctly, have judged it simpler and more intelligible than the nature of surfaces and lines, as is shown by the fact that they explain line as motion of a point and surface as the motion of a line. The Philosophers also posit many motions which they believe can occur without any body s changing place, such as those they call motus ad formam, motus ad calorem, motus ad quantitatem (motion with respect to form, motion with respect to heat, motion with respect to quantity) and countless others. For my own part, I know of no motion other than that which is easier of conceive of than the lines of geometers, by which bodies pass from on place to another and successively occupy all the spaces in between. 27

28 By the time of the Principles (1644), Descartes is confident enough to condense this to: What movement is in the ordinary sense. However, movement (and I mean local movement, because I can conceive no other kind, and because I consequently think that no other should be imagined in the nature of things), as commonly interpreted, is nothing other than the action by which some body travels from one place to another. 28

29 Descartes s intelligible cosmos built on a foundation of clear and distinct ideas All knowledge of the material world is based on clear and distinct ideas The material world consists of matter in motion The sole essential property of matter is extension Motion is local motion (we ll need to ask about Descartes s definition of local motion later) The parts of matter (= bodies) move according to three simple laws of nature (coming up later) From these basic principles, everything about the material world can be explained... 29

30 5.5 The cosmology of The World (unpublished in Descartes s lifetime) See extracts from The World 1. The Aristotelians used the four elements earth, air, fire, and water and their properties of being heavy or light, hot or cold, wet or dry, to explain the world around us. What does Descartes suggest that we use instead? 2. Which body is at the center of Descartes s cosmos? 3. Does the Earth move, according to Descartes? 4. Would you say, on the basis of what Descartes writes in The World, that he is supporting the Copernican point of view? 5. Which year was Galileo convicted by the Inquisition? (this will be important later) 30

31 Descartes s cosmos: overview matter extending indefinitely in all directions swirling vortices carrying planets around stars (such as our sun) at the center of the vortices, at varying distances from our vortext comets passing from one region to another 31

32 Comparison of Cartesian cosmology in The World with Aristotelian cosmology Basic elements and their properties Vacuum? Main route to knowledge What at center? Earth at rest or in motion? One center or multiple centers of rotation? Size and shape Aristotelian cosmology Cartesian cosmology (The World) Terrestrial/celestial distinction? 32

33 Recall these questions from earlier: 3. Does the Earth move, according to Descartes (in The World)? Yes! 4. Would you say, on the basis of what Descartes writes in The World, that he is supporting the Copernican point of view? Yes! 5. Which year was Galileo convicted by the Inquisition? On hearing of Galileo s condemnation, Descartes decides not publish The World. One important reason: fear of controversy. So what did he do? 33

34 Recall these questions from earlier: 3. Does the Earth move, according to Descartes? Yes! 4. Would you say, on the basis of what Descartes writes in The World, that he is supporting the Copernican point of view? Yes! 5. Which year was Galileo convicted by the Inquisition? On hearing of Galileo s condemnation, Descartes decides not publish The World. One important reason: fear of controversy. He took two actions: (1) He went back to the epistemological foundations of his cosmology and tried to make a stronger case The World contains a brief discussion of the unreliability of the senses and then launches into his new cosmology of matter in motion. The Meditations, published in 1641, walk us through, step by step, why we should reject beliefs based on the senses as the starting point for cosmology, and start instead from considerations of reason (from things we can know clearly and distinctly by reflecting on the contents of our own minds. In the Principles, published in 1644, Descartes takes his time in developing the epistemological foundations of his project, before moving on to his cosmology. (2) He sought to avoid the claim that the Earth is in motion. 34

35 5.6 A new theory of motion What is local motion? Local motion is motion from place to place How are places defined? Aristotelian cosmos: with respect to the center of the cosmos So local motion, for Aristotelians, is motion relative to the center of the cosmos Can Descartes use this definition of motion? 35

36 Descartes: a new theory of motion In Descartes s cosmos, when a body moves, what does it move relative to? What does Descartes have in his cosmos that he can use for defining motion? 36

37 Descartes: a new theory of motion In Descartes s cosmos, when a body moves, what does it move relative to? What does Descartes have in his cosmos that he can use for defining motion? Descartes s definition of motion (Principles, Part II.25): What movement properly speaking is. If, however, we consider what should be understood by movement, according to the truth of the matter rather than in accordance with common usage (in order to attribute a determinate nature to it): we can say that it is the transference of one part of matter or one body, from the vicinity of those bodies immediately contiguous to it and considered as at rest, into the vicinity of [some] others. 37

38 Descartes: a new theory of motion Descartes s definition of motion (Principles, Part II.25): What movement properly speaking is. If, however, we consider what should be understood by movement, according to the truth of the matter rather than in accordance with common usage (in order to attribute a determinate nature to it): we can say that it is the transference of one part of matter or one body, from the vicinity of those bodies immediately contiguous to it and considered as at rest, into the vicinity of [some] others. This is the most natural definition of motion, given the resources of Descartes s system This definition solves the problem of the Earth s motion in the Copernican system (see Principles III.26, 30, 33) 38

39 Descartes s intelligible cosmos built on a foundation of clear and distinct ideas All knowledge of the material world is based on clear and distinct ideas The material world consists of matter in motion The sole essential property of matter is extension Motion is local motion as defined in Principles II.25 The parts of matter (= bodies) move according to three simple laws of nature (coming up now...) From these basic principles, everything about the material world can be explained... 39

40 5.7 Descartes s laws of nature (Principles of Philosophy, 1644) 0. A general conservation law Principles II.36: That God is the primary cause of motion; and that He always maintains an equal quantity of it in the universe. God is the general and first cause of all movements in the world God is immutable in His nature and also in the way that He acts. Therefore God maintains all matter exactly as it was at creation, and subject to the same laws as it was at creation. Problem: But we see changes happening around us all the time, so how are these changes consistent with God being immutable and maintaining matter exactly as it was at creation? Solution: He maintains the total quantity of motion, but this total quantity of motion is redistributed among the parts of matter The motion is redistributed according to the laws of nature: these are the particular and secondary causes of motion. This general conservation law for the total quantity of motion precedes the particular laws, which are as follows 40

41 Descartes s laws of nature 1, 2, and 3 Principles II.37: from this same immutability of God, we can obtain knowledge of the rules or laws of nature, which are the secondary and particular causes of the diverse movements which we notice in individual bodies. 1 Each and every thing, in so far as it can, always continues in the same state; and thus what is once in motion always continues to move. 2 All motion is in itself rectilinear. 3 If a body collides with another body that is stronger than itself, it loses none of its motion; but if it collides with a weaker body, it loses a quantity of motion equal to that which it imparts to the other body. 41

42 Descartes Laws of Nature (Principles of Philosophy, 1644) 1 Each and every thing, in so far as it can, always continues in the same state; and thus what is once in motion always continues to move. 2 All motion is in itself rectilinear. 3 If a body collides with another body that is stronger than itself, it loses none of its motion; but if it collides with a weaker body, it loses a quantity of motion equal to that which it imparts to the other body. Newton s Principia (1687) Law I Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. Law II The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. Law III To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. 42

43 5.8 The cosmology of the Principles of Philosophy Descartes s intelligible cosmos built on a foundation of clear and distinct ideas All knowledge of the material world is based on clear and distinct ideas The material world consists of matter in motion The sole essential property of matter is extension Motion is local motion as defined in Principles II.25 The parts of matter (= bodies) move according to three simple laws of nature From these basic principles, everything about the material world can be explained... 43

44 Descartes, Principles of Philosophy (IV: 187) And certainly, if anyone will consider how marvelous are the properties of the magnet and of fire and the remaining things whose causes I have deduced in this piece of writing from principles known and accepted by all (namely from the figure, magnitude, situation, and motion of particles of matter): he will easily be persuaded that there are, in rocks or plants, no forces so secret, no marvels of sympathy or antipathy so astounding, and finally, no effects in all of nature which are properly attributed to purely physical causes or causes lacking in mind and thought; the reasons for which cannot be deduced from these same principles. Consequently, it is unnecessary to add anything else to them. 44

45 Descartes s cosmos in the Principles of Philosophy size and overall structure basic constituents theory of planetary motion: vortices does the Earth move? theory of tides (Principles IV.49-56) theory of terrestrial gravitation (Principles IV.23-28) 45

46 5.9 Review Cartesian cosmology: essential points 1. The Aristotelians used the four elements of earth, air, fire and water to explain the behavior of all inanimate terrestrial objects. Descartes rejects these elements as the starting point for his cosmology. Why? 2. What does Descartes suggest instead, and why? 3. In Aristotelian cosmology, all motion is motion with respect to what? 4. Why can t Descartes use this account of motion in his cosmology? 5. In the Principles, what is Descartes s definition of motion properly speaking? 6. How does Descartes apply the definition of motion given in the Principles to the motion (or non-motion) of the Earth?

47 Cartesian cosmology: essential points 7. The Aristotelian cosmos contains a fundamental distinction between the terrestrial and celestial realms. Is this distinction found in Cartesian cosmology? 8. What are the natural motions of bodies in Aristotelian cosmology? 9. What is Descartes s first law of nature, as stated in the Principles? 10. By what means can a body change its state, according to Descartes? 11. Aristotelian cosmology explains the planetary motions by appeal to the natural motion of celestial bodies. Can Descartes make use of the same explanation? 12. What explanation does Descartes offer for planetary motion? 13. Aristotelian cosmology explains the behavior of falling bodies (terrestrial gravitation) by appeal to the natural motions of the terrestrial elements. Descartes cannot make use of the same explanation. Roughly speaking, what alternative does he offer? 14. Finally, recall that Galileo offered an (erroneous) theory of the tides and used this to argue for the motion of the Earth (as part of his support for the Copernican system). Roughly speaking, what is Descartes s theory of the tides and how does it differ from that of Galileo?