We know nothing whatever of the reason why bodies possess weight. Bodies
do not fall on account of the law of gravitation (§ 9); nor does their
gravity explain why they fall. Gravity, as we have seen, is only a name
for weight, and the law of gravitation is only a statement of =how=
bodies approach one another, not =why= they do so.
It is often said that gravitation is =attraction=, and that bodies fall
to the earth because the earth attracts them. But the word “attract”
simply means to “draw towards,” and “attraction” means nothing but
“drawing towards;” and to say, when two bodies move towards one another,
that they are “drawn towards” one another, is simply to describe the
fact and makes us no whit wiser than we were before. On the contrary,
unless we take great care, it may make us a little less wise. For the
words “drawing towards” are so closely associated with ropes and hooks
and the act of pulling, that we are easily led to fancy the existence of
some analogous invisible machinery in the case of mutually attractive
bodies.
Again, gravitation is spoken of as a =force=; and as the word force is
in very common use, let us try to make out what we mean by it. A man is
said to exert force when he pushes or pulls anything so as either to
exert pressure upon it or to put it in motion. A wrestler’s force is
proved by his hug; a bowler’s force is shown by the swiftness of motion
of the ball.
Force, then, is the name which we give to that which causes or, in the
case of pressure, tends to cause, motion. The force of gravity therefore
means the cause of the pressure which we feel when bodies which possess
gravity are supported by our bodies, and the cause of their movement
towards the centre of the earth, when they are free to move. But it is
exactly about the cause of these phenomena that we know nothing
whatever.
A good deal of mischief is done by the inaccurate use of such words as
attraction and force, as if they were the names of things having an
existence apart from natural objects, and from the series of causes and
effects which are open to our observation; while they are, in reality,
merely the names of the unknown causes of certain phenomena. And it is
worth while to take pains to get clear ideas on this head at the outset
of the study of science.
Let us remember then that, so far as we know, it is a law of nature,
that any two material bodies, if they are free to move, approach one
another with gradually increasing swiftness; and that the space over
which each travels before the two meet, is inversely proportional to the
quantity of matter which it contains. =Attraction of gravitation= is a
name for this general fact; =weight= is the name for the fact in the
case of terrestrial bodies; =force= is a name which we give to the
unknown cause of the fact. The fact is that which it is important to
know. The names are of no great consequence so long as we recollect that
they are merely names and not things.