pulling it towards the earth, or the earth towards it; and yet it is
quite certain that the stone tends to move towards the earth and the
earth towards the stone, in the way defined by the law of gravitation.

In the currently accepted language of science, the cause of motion, in
all such cases as this, when bodies tend to move towards or away from
one or another, without any discernible impact of other bodies, is
termed a 'force,' which is called 'attractive' in the one case, and
'repulsive' in the other. And such attractive or repulsive forces are
often spoken of as if they were real things, capable of exerting a
pull, or a push, upon the particles of matter concerned. Thus the
potential energy of the stone is commonly said to be due to the
'force' of gravity which is continually operating upon it.

Another illustration may make the case plainer. The bob of a pendulum
swings first to one side and then to the other of the centre of the
arc which it describes. Suppose it to have just reached the summit of
its right-hand half-swing. It is said that the 'attractive forces' of
the bob for the earth, and of the earth for the bob, set the former in
motion; and as these 'forces' are continually in operation, they
confer an accelerated velocity on the bob; until, when it reaches the
centre of its swing, it is, so to speak, fully charged with kinetic
energy. If, at this moment, the whole material universe, except the
bob, were abolished, it would move for ever in the direction of a
tangent to the middle of the arc described. As a matter of fact, it
is compelled to travel through its left-hand half-swing, and thus
virtually to go up hill. Consequently, the 'attractive forces' of the
bob and the earth are now acting against it, and constitute a
resistance which the charge of kinetic energy has to overcome. But, as
this charge represents the operation of the attractive forces during