Essentially, Hume doubts the rational foundation of everything that is useful and helps us get by in the world. All action and speculation is based upon suppositions of cause and effect. If I did not think my actions would have any consequences, I would not act. For instance, I go to work because I know I will get money if I do so, and I help my friend because I know my friend will be better off if I do so. If I had no reason to expect any consequences from my actions, I would have no reason to go to work, to help my friend, or anything else besides.
Hume's line of skepticism cuts to the very heart of our conception of ourselves as rational beings. He makes us question what we mean when we say we do things for a reason. In fact, his argument seems to imply, what we call our reasons are not reasons at all, or at least they are not rationally justified.
This brings us to the naturalist line in Hume's philosophy. While Hume denies that we have reasons for believing or acting as we do, he also explains the causes for our behavior and our actions. He argues that induction and causal reasoning are implanted in us by custom and constant conjunction. In replacing reason with custom, Hume reconceives the nature of human thought and action. Most philosophy, in particular the rationalist philosophy of Descartes, sees human beings as primarily rational animals, informed and guided by reason. Hume's reconception sees us more as creatures of custom and habit, much like the animals we so frequently try to set ourselves above.
While Hume's discussion of custom and constant conjunction may seem odd to us, it is in fact just another way of framing something that should be relatively clear. Both Hume and the traditional philosopher would agree that certain events invariably follow certain other events, and both Hume and the traditional philosopher would agree that our behavior is largely dictated by our knowledge of this sequence. The difference lies in the fact that the traditional philosopher would then argue that there is some principle of cause and effect that we know and can see in operation between two connected events. Hume denies that we know any such principle, suggesting instead that habit simply implants an expectation in us that events will fall out in a certain pattern. He uses the term "constant conjunction" to suggest that we cannot say that two events are causally related, but only that we constantly find one followed by the other.
The harsh limitations that Hume sets upon reason might lead us to question the validity of science and the scientific method that Hume holds so dear. An interesting and significant point is that only philosophy, and not science, ever makes any claim regarding the certainty of causal reasoning. All scientific knowledge comes from experience, but science is also careful never to assert the certainty of this knowledge. For instance, Newton's three laws all come from induction: he observes that certain events invariably follow upon one another, and draws up laws to explain this constant conjunction. However, all laws of physics are nothing more than hypotheses. A physical theory, we could argue, can never be proved, but only disproved. Every piece of evidence in its favor only serves to make it more likely, but no evidence could ever make it certain.
Only philosophy, in its yearning for certainty, has tried to suggest that there is such a thing as a law of cause and effect. Science rests content in making predictions based on experience without claiming any kind of certainty or privileged reasoning to back these predictions up. Hume might then also defend his own philosophy, saying that he proceeds according to a similar method.