Hume: Sec IV

Hume believed that there are two different kinds of thinking and humans must keep these two types distinct in the mind. When humans fail to do this that is when people begin to make errors in their thinking. The two types of thinking are “relation of ideas” and “matter of facts”. “Relation of ideas demonstrates a truth “a priori”. Hume explains how mathematicians use relation of ideas constantly. If one knows what a circle and a straight line are, one is likely to agree with the theorems of Euclid’s geometry. If you disagree than it is a self-contradiction. For instance, if a man is a husband, he must have a wife. Just by knowing that a man is a husband, you can infer he has a wife. If you do not then you are being contradictory. The same goes for other words, such as being a mother or father infers that that individual has children. This is a priori, since you know the information before actually seeing or experiencing it. However, one does not know whether there are actual husbands, wives or circles in the world unless you go out and look for it. Thus, to determine their existence you are not using “relation of ideas” but rather “matter of facts”. “Matter of facts” can only be discovered as false or true by observation and not by just looking at the implications of a word. “Matter of facts” cannot be self-contradictory. As Hume states, “That the sun will not rise tomorrow is no less intelligible a proposition and implies no more contradiction than the affirmation, that it will rise.” (Sec IV. Part 1)
Hume’s general proposition is that all cause and effect knowledge is not from a priori, but from experience or a posteriori. One example of this proposition is how Adam “could not have inferred from the fluidity and transparency of water that it would suffocate him, or from the light and warmth of fire that it would consume him.” (Sec. IV Part 1) Thus, only through experience are cause and effects discovered and not by reason. The effect is not related to the cause of any event and you cannot infer anything unless you have experienced it. Just seeing an object does not give us any clues to what will happen when we throw it or step on it. What will the effect be? One cannot know a priori.