well-known warning against the 'absurdity' of supposing that one body
can act on another at a distance through a vacuum, the ultimate
particles of matter were generally assumed to be the seats of
perennial causes of motion termed 'attractive and repulsive forces,'
in virtue of which, any two such particles, without any external
impression of motion, or intermediate material agent, were supposed to
tend to approach or remove from one another; and this view of the
duality of the causes of motion is very widely held at the present
day.

Another important result of investigation, attained in the seventeenth
century, was the proof and quantitative estimation of physical
inertia. In the old philosophy, a curious conjunction of ethical and
physical prejudices had led to the notion that there was something
ethically bad and physically obstructive about matter. Aristotle
attributes all irregularities and apparent dysteleologies in nature to
the disobedience, or sluggish yielding, of matter to the shaping and
guiding influence of those reasons and causes which were hypostatised
in his ideal 'Forms.' In modern science, the conception of the
inertia, or resistance to change, of matter is complex. In part, it
contains a corollary from the law of causation: A body cannot change
its state in respect of rest or motion without a sufficient cause.
But, in part, it contains generalisations from experience. One of
these is that there is no such sufficient cause resident in any body,
and that therefore it will rest, or continue in motion, so long as no
external cause of change acts upon it. The other is that the effect
which the impact of a body in motion produces upon the body on which
it impinges depends, other things being alike, on the relation of a
certain quality of each which is called 'mass.' Given a cause of
motion of a certain value, the amount of motion, measured by distance