After his three-volume Treatise of Human Nature dropped like a rock to the bottom of the pool of British philosophic writing, Hume set out to write a briefer, more accessible version -- the Enquiry concerning Human Understanding. One of the early points it makes is that most endeavors to write about the nature of thought are hopeless and nearly impossible to understand. With that disclaimer, Hume sets out to contradict himself by writing lucidly about, while candidly acknowledging the severe limits of, this topic. He uses logic to show that most human understanding falls into two categories: a very small group of innate truths deducible by logic, like every triangle has three sides, and a much larger group -- nearly everything we "know" -- which is based on reality-based observation. This latter group always has, at a fundamental level, an element of probabilistic assumption: Things customarily happened this way before, so they probably will again. Thus almost everything we (think we) know about the world is based on empirical experience, not pure logic. A must-read for anyone with even a passing interest in philosophy or modern thought, An Enquiry Concerning Human Understanding is primarily concerned with the psychological question of the origin of our concepts. About the answer to this question, the empiricists were all agreed--our concepts are furnished by experience, which includes both sensory experience and introspection (i.e., the experience of our own mental states). The empiricists also agreed about the way we can justify our beliefs. Some beliefs are true (or false) in virtue of the ideas they contained, and we can know their truth (or falsity) simply by thinking about them; other beliefs are true (or false) in virtue of how the external world is, and we can know their truth (or falsity) only by drawing on our experiences of the world. According to Hume, all substantial conclusions about the world fall into this second category. That is, the truth (or falsity) of all substantial claims about the existence and nature of things in the external world can be discovered only by checking those claims against the evidence of our senses.