A Treatise of Human Nature

Hume's first and largest work, published in three books while he was still in his twenties, and subtitled an attempt to introduce the experimental method of reasoning into moral subjects. Book 1, Of the Understanding, divides perceptions into impressions and ideas, holds that ideas copy impressions, and explains thought through the association of ideas. Its most famous part analyses cause and effect: we never observe a necessary connection, only constant conjunction, and our expectation that the future will resemble the past rests on custom, not reason. Book 1 ends in scepticism about the senses, about reason and about the self, which Hume describes as a bundle of perceptions. Book 2, Of the Passions, treats pride, love and the will, and argues that reason alone cannot move us to act; it is the slave of the passions. Book 3, Of Morals, holds that moral distinctions come from sentiment rather than reason, contains the famous passage on deriving 'ought' from 'is', and explains justice as an artificial virtue.

It is where modern debates about causation, induction, the self and the gap between facts and values begin, in Hume's own fullest version.