An Enquiry Concerning Human Understanding begins with the author distingushing the difference between impressions and ideas. Impressions are sensory, emotional, and other types of vivid imagery. Ideas are thoughts, beliefs, or memories directly related to these impressions.
Furthermore, Hume exemplifies the distinctions between ideas and matters of fact. He poses that we understand matters of fact predominantly through the observation of cause a effect. There is a suggestion made that we cannot justify these observations. Hume also suggests that we cannot also justify future predictions from past experiences without the principle that the future will always resemble the past. Our inferences that have to do with matters of fact are based almost entirely off of probability.
Much of the novel discusses the analagous relationship between animal and human thoughts.