The fourth chapter is split into two sections. The first divides objects that the human mind can apply itself to into two categories: “Relations of Ideas, and Matters of Fact” (18). Relations of ideas concern fields such as mathematics, and knowledge that is “intuitively or demonstratively certain” (18). This knowledge is gained and affirmed regardless of experience, sensation, or circumstance. It is concerned with things that are true necessarily and cannot be otherwise. Matters of fact, on the other hand, can be known and experienced, yet could conceivably be otherwise.

Matters of fact seem to be founded on the relation between cause and effect. When asked why one holds a certain belief, a reason will be given; every fact is known in this way. When a new object is presented to the human senses, nothing at all about its effects can be inferred: The human mind can only know through experience. As Hume points out, “causes and effects are discoverable, not by reason, but by experience” (20).

The second part of the chapter discusses the problem of induction. Hume observes that knowledge gained by sense experience is not founded on reasoning. There is nothing about our past experience that necessitates its application to the future.