Hume’s An Enquiry Concerning Human Understanding is his moral philosophy or “science of human nature.” He distinguishes between two types of philosophy. One of them “considers man as chiefly born for action and influenced in his measures by measures and taste and sentiment, pursuing one object and avoiding another according to the value which these objects seem to possess.” Hume seems to be speaking of a practical philosophy. The other philosophy “consider man as a reasonable rather than an active being and endeavor to form his understanding more than cultivate his manners.” Hume is referring to theoretical philosophy. He seems to be saying only the latter type of philosophy is based on reason. This shows his view of reason is more limited than the understanding of it by Medieval philosophers. This point is confirmed when he says: “Man is a reasonable being and as such, receives from his science proper food and nourishment. But so narrow are the bounds of human understanding that little can be hoped for” in the living of daily life. He believes that man is a reasonable being, a social being, and an active being. It seems the intellect only applies to being a reasonable being. He thinks that reason touches only a small part of life. For example, he says, “Be a philosopher, but, amid all your philosophy, be still a man.” Hume is saying that the practical necessities of life still require you to pay heed to them as opposed to being consumed with esoteric thought removed from the ordinariness of everyday life.
Hume seems to follow Locke in his ideas about the origin of ideas. He is a strong empiricist like Locke. He thinks the mind copies ‘the perceptions of the senses” to form its ideas. He thinks, however, that ideas are weaker than the original senses. He does think that the impressions from the senses are a faithful copy of external objects. Hume does not think we have any innate ideas. He thinks all of our ideas come via the senses. He is quite skeptical of metaphysical ideas since they do not come from the senses. This is due to his being a strict empiricist allowing no other means by which one can claim knowledge. He envisions himself as being more of an empiricist than Locke.
Hume thinks there are only two objects of knowledge: “relations of ideas and matters of fact.” Only the first is “demonstratively certain;” the other is only probable. He reflects doubt on the connection of cause and effect. Cause and effect is not discovered by reason, but by experience. Therefore, arguments from cause and effect are only probable because they are not substantiated via our senses. He thinks that we know these things through “custom.” He states that “all inferences from experience, therefore, are effects of custom, not of reasoning.” This confirms what we said earlier about how Hume separates practical reasoning from theoretical reasoning and does not even consider it reason. He sees the knowledge from custom as probable, but he does think it is a “great guide of human life. It is that principle alone which renders our experience useful to us and makes us expect, for the future, a similar train of events with those that have appeared in the past.” Hume adds that “Without the influence of custom we should be entirely ignorant of every matter of fact beyond what is immediately present to the memory or senses.” It seems that custom serves a similar role for Hume as practical wisdom served for Aristotle.
Hume thinks knowledge comes only from the senses or are implied in the definition. He states, “If I ask, why you believe any particular matter of fact which you relate, you must tell me some reason; and this reason will be some other fact connected with it. But as you cannot proceed after this manner in infinitum, you must at last terminate in some fact which is present to your memory or senses or must allow that your belief is entirely without foundation.” Based on this statement, Hume would think of religious belief or metaphysics without foundation. It cannot be considered knowledge or reliable.
 David Hume, An Enquiry Concerning Human Understanding in Classics of Western Philosophy edited by Steven M. Cahn (Indianapolis: Hackett, 2012), 834.
 David Hume, Enquiry Concerning Human Understanding, 834.
 Ibid., 835.
 Hume, Enquiry, 842.
 Ibid., 843.
 Ibid., 849.
 Ibid., 849-850.
 Ibid, 850.