David Hume

An Enquiry Concerning Human Understanding. David Hume

Hume begins by distinguishing between impressions and ideas. Impressions are sensory impressions, emotions, and other vivid mental phenomena, while ideas are thoughts or beliefs or memories related to these impressions. We build up all our ideas from simple impressions by means of three laws of association: resemblance, contiguity, and cause and effect. Next, Hume distinguishes between relations of ideas and matters of fact. Relations of ideas are, for the most part, mathematical truths, so denial of them would result in a contradiction. Matters of fact are the more common truths that we learn from experience. Denying a matter of fact is not contradictory. For the most part, we understand matters of fact according to cause and effect, where a direct impression will lead us to infer some unobserved cause. For instance, I know the sun will rise tomorrow based on past observations and my understanding of cosmology, even though I have yet to observe this fact directly.