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 neither the infallible nor the sole causes of an union among ideas.
They are not the infallible causes.