24. This proposition, that causes and effects are discoverable,
not by reason but by experience, will readily be admitted with
regard to such objects, as we remember to have once been altogether
unknown to us; since we must be conscious of the utter inability,
which we then lay under, of foretelling what would arise from them.
Present two smooth pieces of marble to a man who has no tincture of
natural philosophy; he 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.
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