Theory of Knowledge

Even though an ultimate explanation of both the subject or object of knowledge is impossible, Hume provides a description of how man senses and understands. He emphasizes the utility of knowledge as opposed to its correctness and suggests that experience begins with feeling rather than thought. He uses the term "perception" in its traditional sense - that is, whatever can be present to the mind from the senses, passions, thought, or reflection. Nonetheless he distinguishes between impressions which are felt and ideas which are thought. In this he stresses the difference between feeling a toothache and thinking about such a pain, which had been obscured by both rationalists and empiricists. Both impressions and ideas are subdivided further into simple and complex; for example, the idea of heat is simple, while the idea of combustion is complex.

These simple divisions are the basis for Hume's "phenomenalism" (that is, knowledge consists of "appearances" in the mind). Hume distinguishes the various operations of the mind in a descriptive psychology, or "mental geography." Impressions are described as vivacious and lively, whereas ideas are less vivid and, in fact, derived from original impressions. This thesis leads to the conclusion that "we can never think of any thing which we have not seen without us or felt in our own minds." Hume often overestimates the importance of this discovery with the suggestion that the sole criterion for judging ideas is to remove every philosophical ambiguity by asking "from what impression is that supposed idea derived." If there is no corresponding impression, the idea may be dismissed as meaningless. This assumption that all ideas are reducible, in principle, to some impression is a primary commitment of Hume's empiricism. Hume did admit that there are complex ideas, such as the idea of a city, that are not traceable to any single impression. These complex ideas are produced by the freedom of the imagination to transform and relate ideas independently of impressions; such ideas are not susceptible to empirical verification. This represents the major paradox of Hume's philosophy - the imagination which produces every idea beyond sensible immediacy also denies the truth of ideas.