20. All the objects of human reason or enquiry may naturally be
divided into two kinds, to wit, Relations of Ideas, and Matters of
Fact. Of the first kind are the sciences of Geometry, Algebra, and
Arithmetic; and in short, every affirmation which is either
intuitively or demonstratively certain. That the square of the
hypothenuse is equal to the square of the two sides, is a
proposition which expresses a relation between these figures. That 

three times five is equal to the half of thirty, expresses a
relation between these numbers. Propositions of this kind are
discoverable by the mere operation of thought, without dependence on
what is anywhere existent in the universe. Though there never were a
circle or triangle in nature, the truths demonstrated by Euclid
would for ever retain their certainty and evidence.

22. All reasonings concerning matter of fact seem to be founded on
the realtion of Cause and Effect. By means of that relation alone we
can go beyond the evidence of our memory and senses. If you were to
ask a man, why he believes any matter of fact, which is absent; for
instance, that his friend is in the country, or in France; he would
give you a reason; and this reason would be some other fact; as a
letter received from him, or the knowledge of his former resolutions
and promises.

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.

23. 

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.

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.

25. 

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.

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
ultimate springs and principles are totally shut up from human

curiosity and enquiry. Elasticity, gravity, cohesion of parts,
communication of motion by impulse; these are probably the ultimate
causes and principles which we shall ever discover in nature; and we
may esteem ourselves sufficiently happy, if, by accurate enquiry and
reasoning, we can trace up the particular phenomena to, or near to,
these general principles.