same, though several times absent from and present to the senses; and
ascribe to it an identity, notwithstanding the interruption of the
perception, whenever we conclude, that if we had kept our eye or hand
constantly upon it, it would have conveyed an invariable and
uninterrupted perception. But this conclusion beyond the impressions of
our senses can be founded only on the connexion of cause and effect; nor
can we otherwise have any security, that the object is not changed upon
us, however much the new object may resemble that which was formerly
present to the senses. Whenever we discover such a perfect resemblance,
we consider, whether it be common in that species of objects; whether
possibly or probably any cause coued operate in producing the change and
resemblance; and according as we determine concerning these causes and
effects, we form our judgment concerning the identity of the object.

Here then it appears, that of those three relations, which depend not
upon the mere ideas, the only one, that can be traced beyond our senses
and informs us of existences and objects, which we do not see or feel, is
causation. This relation, therefore, we shall endeavour to explain fully
before we leave the subject of the understanding.

To begin regularly, we must consider the idea of causation, and see from
what origin it is derived. It is impossible to reason justly, without
understanding perfectly the idea concerning which we reason; and it is
impossible perfectly to understand any idea, without tracing it up to its
origin, and examining that primary impression, from which it arises. The
examination of the impression bestows a clearness on the idea; and the
examination of the idea bestows a like clearness on all our reasoning.

Let us therefore cast our eye on any two objects, which we call cause
and effect, and turn them on all sides, in order to find that impression,