Hume acknowledges that the skepticism employed in the previous section could never undermine our reasoning from common life: nature always wins out against abstract reasoning. However, he does claim to have shown that there is a step in our reasoning from experience that is not supported by any argument or process of understanding. There is no solid reason why we should reason according to cause and effect, and yet we never fail to do so.
Hume notes that someone thrown into the world with no prior experience would have no understanding of the process of cause and effect. Life would be an unintelligible string of unconnected events. We cannot sense causation, nor (as Hume has argued in the previous section) is it present to our reason. Hume's answer is that our inductive reasoning regarding experience is derived from custom and not from the understanding. This is why we need to see a process recur many times before we can begin to see two events in the process as causally connected. I need only examine the diagram of one circle in order to derive through reason the properties that all circles share in common. However, we must see many collisions of billiard balls and other objects before custom can implant in us the inference that the movement of one object is causally connected to the movement of another.
Without custom, Hume remarks, reasoning that concerns matters of fact could not extend beyond memory and present sense experience. We could not speculate nor even act if custom had not implanted in us the ability to see certain actions as having certain consequences. Nonetheless, Hume points out, all reasoning from experience ultimately falls back upon simple impressions. What I know about past ages might come from reading a history book, or what I speculate about the future might ultimately fall back upon observations I am making in the present. Our speculations about unobserved matters of fact rest upon a constant conjunction with our present impressions.
Hume suggests that we make inferences by means of the imagination, but draws a careful distinction between fiction and belief. Fiction is the product of pure imagination by means of which we can conjure up all sorts of strange images derived from our simple impressions, such as unicorns, alien civilizations, and what have you. Belief is a combination of imagination and a certain sentiment that we cannot control that suggests to us that our imaginings correspond with reality. When some memory or sense impression is present to our minds, the force of custom will then carry the imagination to think of something to which that impression is constantly conjoined. This force of custom forms our beliefs, and creates a more vivid, forceful, and firm version of our pure imaginings.
Cause and effect, like the other two laws of association discussed in section III, allow the mind to move from one thought to another. When these laws of association are led by custom, they form very strong instinctive beliefs. Hume remarks that it is fitting that our knowledge of causation should be formed by instinct rather than by reason. It is very important that we see the world causally, since it is the source of all action and speculation, and reason is too unreliable a tool. Young children have still unformed reasoning abilities, and even intelligent adults commit countless errors in their reasoning. The instincts enforced by custom are far less susceptible to error, and are thus a far stronger means of securing our knowledge of cause and effect.
The terms "skepticism" and "naturalism" are frequently mentioned in discussions of Hume, and his relationship with each is hotly debated. Hume is termed a skeptic on account of the doubts he raises as to the capabilities of reason. The classic account of modern skepticism is found in Descartes' ##Meditations##, in which all knowledge based on sensory experience is cast into doubt. We might read Hume as going even farther, casting our ability to reason inductively into doubt. While Descartes ultimately squirms away from his doubts, Hume sticks to his, claiming that we have no rational justification for anything outside of immediate sensations and a priori reasoning.