Hume performs a balancing act between making skeptical attacks (step 1) and offering positive theories based on natural beliefs (step 2). In the conclusion to Book 1, though, he appears to elevate his skepticism to a higher level and exposes the inherent contradictions in even his best philosophical theories.
He notes three such contradictions. One centers on what we call induction. Our judgments based on past experience all contain elements of doubt; we are then impelled to make a judgment about that doubt, and since this judgment is also based on past experience it will in turn produce a new doubt. Once again, though, we are impelled to make a judgment about this second doubt, and the cycle continues. He concludes that “no finite object can subsist under a decrease repeated in infinitum.”
A second contradiction involves a conflict between two theories of external perception, each of which our natural reasoning process leads us to. One is our natural inclination to believe that we are directly seeing objects as they really are, and the other is the more philosophical view that we only ever see mental images or copies of external objects.
The third contradiction involves a conflict between causal reasoning and belief in the continued existence of matter. After listing these contradictions, Hume despairs over the failure of his metaphysical reasoning:
The intense view of these manifold contradictions and imperfections in human reason has so wrought upon me, and heated my brain, that I am ready to reject all belief and reasoning, and can look upon no opinion even as more probable or likely than another.
He then pacifies his despair by recognizing that nature forces him to set aside his philosophical speculations and return to the normal activities of common life. He sees, though, that in time he will be drawn back into philosophical speculation in order to attack superstition and educate the world.
Hume’s emphasis on these conceptual contradictions is a unique aspect of his skepticism, and if any part of his philosophy can be designated “Humean skepticism” it is this.
Hume rejects extreme skepticism but accepts skepticism in a more moderate form. He associates extreme Pyrrhonian skepticism with blanket attacks on all reasoning about the external world, abstract reasoning about space and time, or causal reasoning about matters of fact. He argues, though, that we must reject such skepticism since “no durable good can ever result from it.” Instead, he recommends a more moderate or Academic skepticism that tones down Pyrrhonism by, first, exercising caution and modesty in our judgments, and, second, by restricting our speculations to abstract reasoning and matters of fact.