Preliminary Remarks and Definitions. 1 1 



motion, and at the same time bears them all together in its orbit 

 round the sun, which perhaps in its turn carries the earth and 

 the whole system of planets toward some distant constellation. 

 Relative rest, therefore, is really the only kind of rest which can 

 actually take place among the objects to which our attention is di- 

 rected. It is at least all that we can ever be certain of observing. 



16. We are hence led to make a similar distinction with 

 respect to motion, and to separate the absolute motions of bodies, 

 considered with reference to immoveable space, from the relative 

 changes of position which may happen among them. These last 

 therefore may be called relative motions, whether that body of 

 the system to which they are referred, be itself in motion or at 

 rest. The changes of place, for example, among the heavenly 

 bodies, which we observe from the surface of the earth, are not 

 absolute but relative motions, because the earth to which we 

 refer them, as a fixed centre, has actually a motion of rotation 

 on its axis and a progressive motion about the sun. Even when 

 by calculation we have inferred from these observations the ac- 

 tual motions of the heavenly bodies as they would appear, if 

 seen from the sun, we cannot affirm positively that these are 

 absolute motions, since it may be that the sun and the whole 

 planetary system have a common motion in space. 



1 7. According to the idea of inertia which we derive from 

 experience, we must regard the state of motion and that of rest, 

 as simple accidents of matter, which it is incapable of imparting 

 to itself, and which it can only receive from without, and which 

 once received, it cannot alter. When therefore we see a 

 body passing from one of these states to the other, we must re- 

 gard this change as produced and determined by the action of 

 external causes. These causes, whatever they may be, are 

 denominated forces. Nature presents us with an infinite number 

 of them which are at least in appearance of different kinds. 

 Such are the forces produced by the muscles and organs of liv- 

 ing animals, the exercise of which, for the most part, depends 

 solely on the will. Such are also the forces of physical agents, 

 as the expansion of bodies by heat, and their contraction by 

 cold, &c. There are moreover others which seem to be inhe- 

 rent in certain bodies, as the attraction of the magnet for iron, 

 and that which is manifested among electrified bodies. 



