Howard Darmstadter in Philosophy Now:
Born May 7, 1711 of respectable parents in the Scottish Lowlands, his early life was outwardly uneventful. After leaving Edinburgh University, he at first contemplated a legal career, and briefly worked as a clerk for a Bristol merchant. But in his late teens Hume was seized by ideas that “opened up to me a new scene of thought.” He decided to become the Newton of the moral sciences.
Newton had shown that all of the material world was governed by the same mechanical laws. Hume’s great project was to base the study of man and society on similar universal principles. Indeed, the Treatise of Human Nature bore the subtitle Being an Attempt to Introduce the Experimental Method of Reasoning into Moral Subjects.
In the Treatise, Hume tried to formulate the laws governing the succession of our thoughts. The result was a long and generally unconvincing exposition of numerous rules said to direct our mental life. But intertwined with this failed attempt at a complete theory of the mind, and at times buried by it, is Hume’s development of the startling implications of a scientific view of man. His two later Enquiries brought these implications powerfully to the fore.
Like most philosophers of his time, Hume conceived of thought as a flow of mental images. Seeing a tree, imagining a tree, or remembering a tree, were all thought to consist of our having a mental image, more vivid for the seen tree, less vivid for the imagined or remembered tree. A sentence like ‘The Earth is round’ would have a certain type of mental image as its meaning, and believing that the Earth is round necessarily involved a vivid mental image of that type. This theory also explained why certain beliefs were logically impossible. For example, a four-sided triangle was logically impossible (and a three-sided triangle logically necessary) because we could not form a mental image of a triangle that did not have three sides. (Try it.) Hume’s disturbing insight from this way of thinking about thinking, was that all our factual and moral beliefs can therefore only be justified in terms of the psychological laws that govern the succession of images in our minds.