will never discover that they will adhere together in such a manner as
to require great force to separate them in a direct line, while they
make so small a resistance to a lateral pressure. Such events, as bear
little analogy to the common course of nature, are also readily
confessed to be known only by experience; nor does any man imagine that
the explosion of gunpowder, or the attraction of a loadstone, could ever
be discovered by arguments _a priori_. In like manner, when an effect is
supposed to depend upon an intricate machinery or secret structure of
parts, we make no difficulty in attributing all our knowledge of it to
experience. Who will assert that he can give the ultimate reason, why
milk or bread is proper nourishment for a man, not for a lion or
a tiger?

But the same truth may not appear, at first sight, to have the same
evidence with regard to events, which have become familiar to us from
our first appearance in the world, which bear a close analogy to the
whole course of nature, and which are supposed to depend on the simple
qualities of objects, without any secret structure of parts. We are apt
to imagine that we could discover these effects by the mere operation of
our reason, without experience. We fancy, that were we brought on a
sudden into this world, we could at first have inferred that one
Billiard-ball would communicate motion to another upon impulse; and that
we needed not to have waited for the event, in order to pronounce with
certainty concerning it. Such is the influence of custom, that, where it
is strongest, it not only covers our natural ignorance, but even
conceals itself, and seems not to take place, merely because it is found
in the highest degree.

25. But to convince us that all the laws of nature, and all the
operations of bodies without exception, are known only by experience,