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?

  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, the following reflections may, perhaps, suffice. Were
any object presented to us, and were we required to pronounce
concerning the effect, which will result from it, without consulting
past observation; after what manner, I beseech you, must the mind
proceed in this operation? It must invent or imagine some event, which
it ascribes to the object as its effect; and it is plain that this
invention must be entirely arbitrary. The mind can never possibly find
the effect in the supposed cause, by the most accurate scrutiny and
examination. For the effect is totally different from the cause, and
consequently can never be discovered in it. Motion in the second
billiard-ball is a quite distinct event from motion in the first;
nor is there anything in the one to suggest the smallest hint of the
other. A stone or piece of metal raised into the air, and left without
any support, immediately falls: but to consider the matter a priori,
is there anything we discover in this situation which can beget the
idea of a downward, rather than an upward, or any other motion, in the
stone or metal?

  And as the first imagination or invention of a particular effect, in
all natural operations, is arbitrary, where we consult not experience;
so must we also esteem the supposed tie or connexion between the cause
and effect, which binds them together, and renders it impossible
that any other effect could result from the operation of that cause.
When I see, for instance, a billiard-ball moving in a straight line
towards another; even suppose motion in the second ball should by
accident be suggested to me, as the result of their contact or
impulse; may I not conceive, that a hundred different events might
as well follow from that cause? May not both these balls remain at
absolute rest? May not the first ball return in a straight line, or
leap off from the second in any line or direction? All these
suppositions are consistent and conceivable. Why then should we give
the preference to one, which is no more consistent or conceivable than
the rest? All our reasonings a priori will never be able to show us
any foundation for this preference.

  In a word, then, every effect is a distinct event from its cause. It
could not, therefore, be discovered in the cause, and the first
invention or conception of it, a priori, must be entirely arbitrary.
And even after it is suggested, the conjunction of it with the cause
must appear equally arbitrary; since there are always many other
effects, which, to reason, must seem fully as consistent and
natural. In vain, therefore, should we pretend to determine any single
event, or infer any cause or effect, without the assistance of
observation and experience.

  26. Hence we may discover the reason why no philosopher, who is
rational and modest, has ever pretended to assign the ultimate cause
of any natural operation, or to show distinctly the action of that
power, which produces any single effect in the universe. It is
confessed, that the utmost effort of human reason is to reduce the
principles, productive of natural phenomena, to a greater
simplicity, and to resolve the many particular effects into a few
general causes, by means of reasonings from analogy, experience, and
observation. But as to the causes of these general causes, we should
in vain attempt their discovery; nor shall we ever be able to
satisfy ourselves, by any particular explication of them. These