When we have made out by careful and repeated observation that something
is always the cause of a certain effect, or that certain events always
take place in the same order, we speak of the truth thus discovered as a
=law of Nature=. Thus it is a law of nature that anything heavy falls to
the ground if it is unsupported; it is a law of nature that, under
ordinary conditions, lead is soft and heavy, while flint is hard and
brittle; because experience shows us that heavy things always do fall if
they are unsupported, that, under ordinary conditions, lead is always
soft and that flint is always hard.
In fact, everything that we know about the powers and properties of
natural objects and about the order of nature may properly be termed a
law of nature. But it is desirable to remember that which is very often
forgotten, that the laws of nature are not the causes of the order of
nature, but only our way of stating as much as we have made out of that
order. Stones do not fall to the ground in consequence of the law just
stated, as people sometimes carelessly say; but the law is a way of
asserting that which invariably happens when heavy bodies at the surface
of the earth, stones among the rest, are free to move.
The laws of nature are, in fact, in this respect, similar to the laws
which men make for the guidance of their conduct towards one another.
There are laws about the payment of taxes, and there are laws against
stealing or murder. But the law is not the cause of a man’s paying his
taxes, nor is it the cause of his abstaining from theft and murder. The
law is simply a statement of what will happen to a man if he does not
pay his taxes, and if he commits theft or murder; and the cause of his
paying his taxes, or abstaining from crime (in the absence of any better
motive) is the fear of consequences which is the effect of his belief in
that statement. A law of man tells what we may expect society will do
under certain circumstances; and a law of nature tells us what we may
expect natural objects will do under certain circumstances. Each
contains information addressed to our intelligence, and except so far as
it influences our intelligence, it is merely so much sound or writing.
While there is this much analogy between human and natural laws,
however, certain essential differences between the two must not be
overlooked. Human law consists of commands addressed to voluntary
agents, which they may obey or disobey; and the law is not rendered null
and void by being broken. Natural laws, on the other hand, are not
commands but assertions respecting the invariable order of nature; and
they remain laws only so long as they can be shown to express that
order. To speak of the violation, or the suspension, of a law of nature
is an absurdity. All that the phrase can really mean is that, under
certain circumstances the assertion contained in the law is not true;
and the just conclusion is, not that the order of nature is interrupted,
but that we have made a mistake in stating that order. A true natural
law is an universal rule, and, as such, admits of no exceptions.
Again, human laws have no meaning apart from the existence of human
society. Natural laws express the general course of nature, of which
human society forms only an insignificant fraction.