The Cambridge History of English and American Literature in 18 Volumes (1907–21).
Volume X. The Age of Johnson.
§ 8. Hume’s Theory of Belief
The discussion of causation brings out another and still more general doctrine held by Hume—his theory of belief. When I say that flame causes heat, I do not refer to a connection of ideas in my own mind; I am expressing belief in an objective connection independent of my mental processes. But Hume’s theory of causation reduces the connection to a subjective routine. Now, some other impression than “flame” might precede the idea of heat—the impression “cold,” for instance. How is it, then, that I do not assert “cold causes heat”? The sequence “cold—heat” may be equally real in my mind with the sequence “flame—heat.” How is it that the former does not give rise to belief in the way that the latter does? Hume would say that the only difference is that the association in the former case is less direct and constant than in the latter, and thus leads to an idea of less force and liveliness. Belief, accordingly, is simply a lively idea associated with a present impression. It belongs to the sensitive, not to the rational, part of our nature. And yet it marks the fundamental distinction between judgment and imagination.
In the Treatise, at any rate, there is no faltering of purpose or weakening of power when the author proceeds to apply his principles to the fabric of knowledge. It is impossible, in this place, to follow his subtle and comprehensive argument; but its issue is plain. With objections not unlike Berkeley’s, he dismisses the independent existence of bodies, and then he turns a similar train of reasoning against the reality of the self: