In the same way, a law of nature, in the scientific sense, is the
product of a mental operation upon the facts of nature which come
under our observation, and has no more existence outside the mind than
colour has. The law of gravitation is a statement of the manner in
which experience shows that bodies, which are free to move, do, in
fact, move towards one another. But the other facts of observation,
that bodies are not always moving in this fashion, and sometimes move
in a contrary direction, are implied in the words "free to move." If
it is a law of nature that bodies tend to move towards one another in
a certain way; it is another and no less true law of nature that, if
bodies are not free to move as they tend to do, either in consequence
of an obstacle, or of a contrary impulse from some other source of
energy than that to which we give the name of gravitation, they either
stop still, or go another way.
Scientifically speaking, it is the acme of absurdity to talk of a man
defying the law of gravitation when he lifts his arm. The general
store of energy in the universe working through terrestrial matter is
doubtless tending to bring the man's arm down; but the particular
fraction of that energy which is working through certain of his
nervous and muscular organs is tending to drive it up, and more energy
being expended on the arm in the upward than in the downward
direction, the arm goes up accordingly.
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