Berkeley, Principles of Human Knowledge

# Someone might define relative motion in such a way

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Unformatted text preview: st abstract from our senses, since it may be that none of those bodies that seem to be at rest are truly so: and a thing that is moved relatively may be really - ·that is, absolutely· - at rest. Similarly, a single body may at one time be both in relative rest and in motion, or even be moving with contrary relative motions, according as its place is variously defined. All this indeterminacy is to be found in the apparent ·or relative· motions, but not at all in the true or absolute ones, and so science should attend only to the latter. True motions, we are told ·by Newton·, are distinguished from apparent or relative ones by the following properties. First, in true or absolute motion, anything that keeps the same position in relation to a whole undergoes any motions that the whole undergoes. Secondly, when a place is moved, anything that is in the place is also moved: so that a body moving in a place that is in motion undergoes the motion of its place. Thirdly, a body never starts to move or changes how it is moving unless a force acts upon it. Fourthly, a body’s true motion is always changed when force acts on it. Fifthly, in circular motion that is merely relative, there is no centrifugal force; but in true or absolute circular motion there is centrifugal force, which is proportional to the quantity of motion. 112. Despite all this, it does not appear to me that there can be any motion except relative motion. To conceive motion, ·it seems to me·, one must conceive at least two bodies that alter in their distance from, or position in relation to, each other. Hence if there was one only body in existence, it could not possibly be moved. This seems obvious, because the idea that I have of motion necessarily includes relation. 113. But although in every motion one must conceive two or more bodies, it can happen that one only is moved, namely the one that is acted on by the force causing the change of distance. Someone might define relative motion in such a way that a body counts as moving if it changes its distance from some other body, even if the force or action causing that change is not applied to it. But ·that would be a bad definition, and here is why·. Relative motion is something we perceived by our senses, something we have to do with in the ordinary affairs of life; so it seems that every man of common sense knows what it is, as well as the best scientist. Now, I ask anyone whether, in this sense of ‘motion’, the 43 stones under his feet move as he walks along the street, because they change their distances from his feet? It seems to me that though motion includes a relation of one thing to another, it is not necessary that each of the related things be said to move. As a man may think of something that does not think, so a body may be moved to or from another body that does not move. 114. As the place of a thing happens to be variously defined, so its motion varies. A man in a ship may be said to be motionless in relation to the sides of the vessel, and yet to move in relation to the land. Or he may move eastward in respect of the ship and westward in respect of the land. In the common affairs of life, men never go beyond the earth to define the place of any body; and what is motionless in respect of that is thought of as absolutely motionless. But scientists, who have a greater extent of thought and more accurate notions of the system of things, have learned that even the earth itself moves. In order therefore to fix their notions, they seem to conceive the material world as finite, and its unmoving outer walls or shell to be the place in terms of which they estimate ‘true motions’. If we consult our own conceptions, I think we shall find that the only idea we can form of absolute motion is basically the idea of relative motion defined in that manner, ·i.e. in terms of relations to the outermost shell of the world·. For, as I have already remarked, absolute motion without external relation is incomprehensible; and all the above-mentioned properties, causes, and effects ascribed to absolute motion will, I think, be found to fit with this ·outer-shell· kind of relative motion. As to what is said ·by Newton· about the centrifugal force, namely that it does not at all belong to circular relative motion: I do not see how this follows from the experiment that is brought to prove it. [Berkeley here gives the reference to Newton’s Principia.] For the water in the vessel, at the time at which it is said to have the greatest relative circular motion, really has no motion at all; as is plain from the foregoing section. ·In the following section I defend this further·. 115. A body does not count as moving unless (1) its distance from, or relation to, some other body alters, and (2) the force or action bringing about that alteration is applied to it ·rather than to the other body·. If either of these is lacking, I do not think that it conforms with how people in general think an...
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## This note was uploaded on 03/12/2013 for the course PHIL 105 taught by Professor Mendetta during the Spring '13 term at SUNY Stony Brook.

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