and the more care we employ, the more assurance shall we acquire, that
the enumeration, which we form from the whole, is complete and entire.

  * Resemblance.

  *(2) Contiguity.

  *(3) Cause and effect.

  *(4) For instance Contrast or Contrariety is also a connexion
among Ideas: but it may, perhaps, be considered as a mixture of
Causation and Resemblance. Where two objects are contrary, the one
destroys the other; that is, the cause of its annihilation, and the
idea of the annihilation of an object, implies the idea of its
former existence.

               Sect. IV. Sceptical Doubts concerning the

                   Operations of the Understanding

                               PART I.

  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.

  21. Matters of fact, which are the second objects of human reason,
are not ascertained in the same manner; nor is our evidence of their
truth, however great, of a like nature with the foregoing. The
contrary of every matter of fact is still possible; because it can
never imply a contradiction, and is conceived by the mind with the
same facility and distinctness, as if ever so conformable to
reality. That the sun will not rise tomorrow is no less intelligible a
proposition, and implies no more contradiction than the affirmation,
that it will rise. We should in vain, therefore, attempt to
demonstrate its falsehood. Were it demonstratively false, it would
imply a contradiction, and could never be distinctly conceived by
the mind.

  It may, therefore, be a subject worthy of curiosity, to enquire what
is the nature of that evidence which assures us of any real
existence and matter of fact, beyond the present testimony of our
senses, or the records of our memory. This part of philosophy, it is
observable, has been little cultivated, either by the ancients or
moderns; and therefore our doubts and errors, in the prosecution of so
important an enquiry, may be the more excusable; while we march
through such difficult paths without any guide or direction. They
may even prove useful, by exciting curiosity, and destroying that
implicit faith and security, which is the bane of all reasoning and
free enquiry. The discovery of defects in the common philosophy, if
any such there be, will not, I presume, be a discouragement, but
rather an incitement, as is usual, to attempt something more full
and satisfactory than has yet been proposed to the public.

  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. A man finding a watch or any other machine in a desert
island, would conclude that there had once been men in that island.
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