Thus not only our reason fails us in the discovery of the ultimate
connexion of causes and effects, but even after experience has informed
us of their constant conjunction, it is impossible for us to satisfy
ourselves by our reason, why we should extend that experience beyond
those particular instances, which have fallen under our observation. We
suppose, but are never able to prove, that there must be a resemblance
betwixt those objects, of which we have had experience, and those which
lie beyond the reach of our discovery.

We have already taken notice of certain relations, which make us pass
from one object to another, even though there be no reason to determine us
to that transition; and this we may establish for a general rule, that
wherever the mind constantly and uniformly makes a transition without any
reason, it is influenced by these relations. Now this is exactly the
present case. Reason can never shew us the connexion of one object with
another, though aided by experience, and the observation of their constant
conjunction in all past instances. When the mind, therefore, passes from
the idea or impression of one object to the idea or belief of another, it
is not determined by reason, but by certain principles, which associate
together the ideas of these objects, and unite them in the imagination.
Had ideas no more union in the fancy than objects seem to have to the
understanding, we coued never draw any inference from causes to effects,
nor repose belief in any matter of fact. The inference, therefore,
depends solely on the union of ideas.

The principles of union among ideas, I have reduced to three general
ones, and have asserted, that the idea or impression of any object
naturally introduces the idea of any other object, that is resembling,
contiguous to, or connected with it. These principles I allow to be