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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
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.
- Spring '13