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An Enquiry Concerning Human Understanding

All our reasonings concerning fact are of the same nature. And here it is constantly
supposed that there is a connexion between the present fact and that which is inferred
from it. Were there nothing to bind them together, the inference would be entirely
precarious. The hearing of an articulate voice and rational discourse in the dark assures us
of the presence of some person: Why? because these are the effects of the human make
and fabric, and closely connected with it. If we anatomize all the other reasonings of this
nature, we shall find that they are founded on the relation of cause and effect, and that
this relation is either near or remote, direct or collateral. Heat and light are collateral
effects of fire, and the one effect may justly be inferred from the other.
23. If we would satisfy ourselves, therefore, concerning the nature of that evidence,
which assures us of matters of fact, we must enquire how we arrive at the knowledge of
cause and effect.
I shall venture to affirm, as a general proposition, which admits of no exception, that the
knowledge of this relation is not, in any instance, attained by reasonings a priori; but
arises entirely from experience, when we find that any particular objects are constantly
conjoined with each other. Let an object be presented to a man of ever so strong natural
reason and abilities; if that object be entirely new to him, he will not be able, by the most
accurate examination of its sensible qualities, to discover any of its causes or effects.
Adam, though his rational faculties be supposed, at the very first, entirely perfect, could
not have inferred from the fluidity and transparency of water that it would suffocate him,
or from the light and warmth of fire that it would consume him. No object ever discovers,
by the qualities which appear to the senses, either the causes which produced it, or the
effects which will arise from it; nor can our reason, unassisted by experience, ever draw
any inference concerning real existence and matter of fact.
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?