What is the theme in A Treatise of Human Nature by David Hume?

Radical Empiricism is a major theme. Hume is one of history's great empiricists. An empiricist is someone who believes that non-trivial knowledge comes only from the senses, denying that reason alone has access to the world as it is. Hume believes that the mind trades only in perceptions, which consist in impressions or sense-data. Hume holds that the mind then generates impressions of representations, ideas and uses them in various ways. All ideas, however, originate in impressions; there are no ideas of pure reason.