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27. Natural Reality / A. Classical Physics / 1. Mechanics / a. Explaining movement

[explaining why not all things are stationary]

30 ideas
Thales must have thought soul causes movement, since he thought magnets have soul [Thales, by Aristotle]
     Full Idea: Thales seems, from what is recorded of him, to have supposed that the soul is something productive of movement, if he really said that the magnet has soul because it produces movement in iron.
     From: report of Thales (fragments/reports [c.585 BCE]) by Aristotle - De Anima 405a20
All things are in a state of motion [Heraclitus, by Aristotle]
     Full Idea: All things are in a state of motion.
     From: report of Heraclitus (fragments/reports [c.500 BCE]) by Aristotle - Topics 104b22
     A reaction: This seems right, I would say. It seems to make a 'process' the fundamental category of ontology, rather than an 'object'.
It is feeble-minded to look for explanations of everything being at rest [Aristotle on Parmenides]
     Full Idea: For people to ignore the evidence of their senses and look for an explanation for everything being at rest is feeble-minded.
     From: comment on Parmenides (fragments/reports [c.474 BCE]) by Aristotle - Physics 253a32
     A reaction: Not exactly an argument, but an interestingly robust assertion of commonsense against dodgy arguments. Aristotle is not exactly an empiricist, but he is on that side of the fence.
That which moves, moves neither in the place in which it is, nor in that in which it is not [Zeno of Elea]
     Full Idea: That which moves, moves neither in the place in which it is, nor in that in which it is not.
     From: Zeno (Elea) (fragments/reports [c.450 BCE], B4), quoted by (who?) - where?
There is no real motion, only the appearance of it [Melissus, by Diog. Laertius]
     Full Idea: There is no such thing as real motion, but there only appears to be such.
     From: report of Melissus (fragments/reports [c.443 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 09.4.3
Practical reason is based on desire, so desire must be the ultimate producer of movement [Aristotle]
     Full Idea: There seem to be two producers of movement, either desire or practical intellect, but practical reason begins in desire.
     From: Aristotle (De Anima [c.329 BCE], 433a12-)
If all movement is either pushing or pulling, there must be a still point in between where it all starts [Aristotle]
     Full Idea: Every movement being either a push or a pull, there must be a still point as with the circle, and this will be the point of departure for the movement.
     From: Aristotle (De Anima [c.329 BCE], 433b24)
If the more you raise some earth the faster it moves, why does the whole earth not move? [Aristotle]
     Full Idea: If you raise some earth and release it, it moves and won't stay put, and the more you raise it the faster it moves, so why does the whole earth not move?
     From: Aristotle (On the Heavens [c.336 BCE], 294a12)
Motion fulfils potentiality [Aristotle]
     Full Idea: Motion is the fulfilment of what exists potentially, in so far as it exists potentially.
     From: Aristotle (Physics [c.337 BCE], 201a10-11), quoted by Rowland Stout - Action 6 'Two'
     A reaction: We might put that as 'all motion is the fulfilment of a natural power'. But that gives the source of motion, and not its intrinsic nature.
If movement can arise within an animal, why can't it also arise in the universe? [Aristotle]
     Full Idea: Movement can arise within a motionless animal out of the object itself, rather than being due to some external agent. But why should this not also be true of the universe?
     From: Aristotle (Physics [c.337 BCE], 252b24)
     A reaction: A nice objection to the Unmoved Mover concept of God. Unfortunately it is ruined by the modern realisation that an animal is never 'motionless', because brain activity is continuous, and ceases only with death.
When there is unnatural movement (e.g. fire going downwards) the cause is obvious [Aristotle]
     Full Idea: Examples of unnatural movements are something earthy moving upwards and fire moving downwards. …When they are moved unnaturally it is obvious what they are moved by, but this is not obvious in the case of their natural movements.
     From: Aristotle (Physics [c.337 BCE], 254b21)
     A reaction: Aristotle always struggles when he tries to give an account of 'unnatural' events. It is hard to see how it is unnatural when a wind blows a flame down, or volcanoes blow earth up. There is hardly a natural distinction of causes which are 'obvious'.
Motion can't move where it is, and can't move where it isn't, so it can't exist [Pyrrho, by Diog. Laertius]
     Full Idea: Motion is not moved in the place in which it is is, and it is impossible that it should be moved in the place in which it is not, so there is no such thing as motion.
     From: report of Pyrrho (reports [c.325 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 09.11.11
If there were no space there could be no movement, or even creation [Lucretius]
     Full Idea: We see movement everywhere, but if there were no empty space, things would be denied the power of movement - or rather, they could not possibly have come into existence, embedded as they would have been in motionless matter.
     From: Lucretius (On the Nature of the Universe [c.60 BCE], I.342)
     A reaction: This still seems a good argument, if reality is made of particles. People can move in a crowd until it becomes too dense.
Atoms move themselves [Lucretius]
     Full Idea: Atoms move themselves.
     From: Lucretius (On the Nature of the Universe [c.60 BCE], II.133)
     A reaction: Something has to move itself, I suppose, but then that could be psuché, giving us free will (see Idea 1424). Why does Epicurus need the 'swerve' if atoms are self-movers? See Idea 5708.
A man walking backwards on a forwards-moving ship is moving in a fixed place [Sext.Empiricus]
     Full Idea: If a ship moves forward and a man carries a rod backwards on it, then it is possible for an object to move without quitting its place.
     From: Sextus Empiricus (Against the Physicists (two books) [c.180], II.056)
     A reaction: [summary of a verbose paragraph] The point is that you cannot define movement as change of place (contrary to Russell's proposal!). The concept of a place seems to be relative. Walking on a treadmill.
Does the original self-mover push itself from behind, or pull itself from in front? [Sext.Empiricus]
     Full Idea: Self-movement must move in some particular direction, but if it pushes it will be behind itself, and if it pulls it will be in front of itself.
     From: Sextus Empiricus (Outlines of Pyrrhonism [c.180], III.68)
     A reaction: This is the same as Aquinas's First Way of proving God's existence.
If time and place are infinitely divided, it becomes impossible for movement ever to begin [Sext.Empiricus]
     Full Idea: If bodies, and the places and times when they are said to move, are divided into infinity, motion will not occur, it being impossible to find anything which will initiate the first movement.
     From: Sextus Empiricus (Outlines of Pyrrhonism [c.180], III.76)
If all atoms, times and places are the same, everything should move with equal velocity [Sext.Empiricus]
     Full Idea: If objects are reducible to atoms, and each thing passes in an atomic time with its own first atom into an atomic point of space, then all moving things are of equal velocity.
     From: Sextus Empiricus (Outlines of Pyrrhonism [c.180], III.77)
Motion is losing one place and acquiring another [Hobbes]
     Full Idea: Motion is privation of one place, and the acquisition of another.
     From: Thomas Hobbes (De Corpore (Elements, First Section) [1655], 1.6.06)
     A reaction: This is basically the 'at-at' theory of motion which empiricists like, because it breaks motion down into atoms of experience. Hobbes needs an ontology which includes 'places'.
Newton reclassified vertical motion as violent, and unconstrained horizontal motion as natural [Newton, by Harré]
     Full Idea: Following Kepler, Newton assumed a law of universal gravitation, thus reclassifying free fall as a violent motion and, with his First Law, fixing horizontal motion in the absence of constraints as natural
     From: report of Isaac Newton (Principia Mathematica [1687]) by Rom Harré - Laws of Nature 1
     A reaction: This is in opposition to the Aristotelian view, where the downward motion of physical objects is their natural motion.
Motion is just change of distance between two things [Locke]
     Full Idea: Motion is nothing but change of distance between two things.
     From: John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.13.14)
     A reaction: If a thing moved steadily relative to other objects, and we then removed all other objects in the universe, would it still be moving?
Maybe motion is definable as 'change of place' [Leibniz]
     Full Idea: I believe 'motion' to be definable, and the definition which says that it is 'change of place' deserves respect.
     From: Gottfried Leibniz (New Essays on Human Understanding [1704], 3.04)
     A reaction: This seems to be the 'at-at' view of motion, championed by Bertrand Russell. (At p1 at t1, at p2 at t2...). Leibniz's version only mentions space and not time, and it includes 'change', which would need definition without mentioning motion.
All that is real in motion is the force or power which produces change [Leibniz]
     Full Idea: As for motion, what is real in it is force or power; that is to say, what there is in the present state which carries with it a change in the future. The rest is only phenomena and relations.
     From: Gottfried Leibniz (Letter to the Editor about Bayle [1698], §13)
     A reaction: This presumably contradicts Newton's concept of inertia, which allows constant motion without force. I always like a reference to powers. What is 'kinetic energy' in this context?
Bodies are recreated in motion, and don't exist in intervening instants [Leibniz]
     Full Idea: I have demonstrated that whatever moves is continuously created and that bodies are nothing at any time between the instants in motion.
     From: Gottfried Leibniz (Letters to Thomasius [1669], 1669.04), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 1
     A reaction: Leibniz is a little over-confident about what he has 'demonstrated', but I think (from this remark) that he would not have been displeased with quantum theory, and the notion of a 'quantum leap' and a 'Planck time'. A 'conatus' is a 'smallest motion'.
Russell's 'at-at' theory says motion is to be at the intervening points at the intervening instants [Russell, by Psillos]
     Full Idea: To reply to Zeno's Arrow Paradox, Russell developed his 'at-at' theory of motion, which says that to move from A to B is to be at the intervening points at the intervening instants.
     From: report of Bertrand Russell (Human Knowledge: its scope and limits [1948]) by Stathis Psillos - Causation and Explanation §4.2
     A reaction: I wonder whether Russell's target was actually Zeno, or was it a simplified ontology of points and instants? The ontology will also need identity, to ensure it is the same thing which arrives at each point.
Occupying a place and change are prior to motion, so motion is just occupying places at continuous times [Russell]
     Full Idea: The concept of motion is logically subsequent to that of occupying as place at a time, and also to that of change. Motion is the occupation, by one entity, of a continuous series of places at a continuous series of times.
     From: Bertrand Russell (The Principles of Mathematics [1903], §442)
     A reaction: This is Russell's famous theory of motion, which came to be called the 'At-At' theory (at some place at some time). It seems to mathematically pin down motion all right, but seems a bit short on the poetry of the thing.
We perceive motion, and not just successive occupations of different positions [Harré/Madden]
     Full Idea: A moving thing is perceptually distinct from a motionless thing, but takes on no new quality. The perception of its motion is a genuine perception. Its motion is not inferred from observation of its successive occupations of different relative positions.
     From: Harré,R./Madden,E.H. (Causal Powers [1975], 3.II)
     A reaction: This seems to be a response to Russell's reductive 'at-at' account of motion, which always struck me as wrong. It doesn't prove Russell wrong, of course, and they are trying to demonstrate that we perceive causation directly.
We only see points in motion, and thereby infer movement [Rescher]
     Full Idea: We perceive motion only as isolated points, and then infer it without actually seeing it.
     From: Nicholas Rescher (Scepticism [1980], §112)
     A reaction: Note how writing suddenly becomes readable as you slow down on entering a railway station. Is that points suddenly becoming unified? This is an empiricist endorsement of Russell's 'at-at' account of motion.
If motion is change of distance between objects, it involves no intrinsic change in the objects [Lowe]
     Full Idea: If motion just is change of distance between two objects, it does not involve any kind of intrinsic change in the objects in question.
     From: E.J. Lowe (A Survey of Metaphysics [2002], p.242)
     A reaction: It sound respectably relativistic, but I doubt the definition. x is moving relative to y, then y attains x's velocity, so x ceases to move? Maybe.
Maybe motion is a dynamical quantity intrinsic to a thing at a particular time [Sider]
     Full Idea: There is an alternative to the Russellian 'at-at' theory of motion, according to which dynamical quantities are intrinsic to times. Whether and how an object is moving at a time is a fact about what that object is like then.
     From: Theodore Sider (Four Dimensionalism [2001], 2.2)
     A reaction: I think I find this quite appealing, because there is too much of a tendency to think of objects as passive and inert, with laws, forces, motions etc. imposed from the outside. But nature is active and dynamic. However, motion can't be wholly intrinsic.