Hume begins to have skeptical doubts about the operations of understanding. He says there are two types of human understanding (only one of these concerns its investigation of what we know to be true or certain). Hume says that all the faculties of human reasoning are divided into two kinds; relationships between ideas and facts. Relationships of ideas are knowledge found in science or mathematics. They are required without experience and can be proven without experience, for example the Pythagorean theorem. Or that three times five is fifteen. Negations of relations of ideas imply a contradiction, for example three times five is twenty. However, Hume is not concerned with relations of ideas because relations of ideas cannot be other than true. Facts (ironically we call them facts, when in reality they are not) that conflict with relations of ideas, can always be refuted and contradicted in our minds. For example, the sun will not rise tomorrow is no more a contradiction than saying the son will rise tomorrow. Questions of fact are based on experience. All reasoning concerning matters of fact are constituted by the relationship of cause and effect on the sensory impression. The knowledge of that cause and effect is not a priori, it comes from the senses. If we see some new object, we will be able to know its causes and effects. When we come into contact with this new object we are not able to automatically know what caused it or what it could cause. This is true for billiard balls. We think we can predict simply by observing one billiard ball hitting another, but we cannot deduce this a priori, without experience. The only reason we think we could or would want to know what happens is because we have…half the card…that we get when we experience said constant conjunction. This “feeling” confirms our belief that this thing exists or is caused by what we believe it is caused by. For example, God, people believe in God because of the feeling they get when a benevolence is presented to them. In Christianity this is called the “holy spirit”. This habit of making connections between events and their supposed causes has helped us survive. When we see someone get punched in the face and then are presented with the same action, we don't stand there and get punched in the face, we assume we're about to get punched and try to run away. of the way. There is no subjective or objective (sensory) impression that connects to necessary causality. Although our idea of ​​causality contains nothing more than a constant conjunction, we will in no way be able to predict the future.