But though I acknowledge this to be a true principlec of association among ideas, I contend that it is the very same as that between the ideas of cause and effect, and is an essential part of all our causal reasonings. The only notion of cause and effect that we have is that of certain objects that have been always conjoined together, and in all past instances have been found inseparable. We can’t penetrate into the reason for that conjunction. We only observe the fact itself: from constant conjunction, objects acquire a union in the imagination. When the impression of one becomes present to us, we immediately form an idea of whatever usually accompanies it; and consequently we can lay this down as one part of the definition of opinion or belief, that it is an idea related to or associated with a present impression.