Hume

There are two types of knowldge:

1. Relations of Ideas: These are a priori knowledge, which are necessarily true and can be known independently of experience. Examples include 1 + 1 = 2 and all bachelors are unmarried. The opposite of these statements is logically impossible—they are contradictory.
2. Matters of Fact: These are a posteriori knowledge, which can only be known through experience. Example: "The sun will rise tomorrow". Its opposite ("the sun will NOT rise tomorrow") is false, but not contradictory—we can conceive it.

Hume thinks relations of ideas are boring.

Knowledge of matters of fact is not universally valid. It is based on our habit of expecting the future to resemble the past, which is not logically justified. This is known as the problem of induction.