David Hume was an anomaly of the Age of Reason. His contemporaries stoutly defended the ability of human beings to use a reason that they held to be divinely inspired to solve the assorted Mysteries of the Universe. For them, the evidence of the senses could only lead mankind away from these Revealed Truths. But Hume, in his An Essay Concerning Human Understanding, countered with equal stoutness, denying the validity of any conclusion not reached by sensory experience. If one could not form impressions by sight, taste, touch, sound, or smell, then the significance of that impression, in Hume's opinion, had to be discounted. Related to the link between impressions gathered by sense and those by reason, Hume posited that the conformity between an image of an object received from the external world and the reality of that object as it truly existed in that external world was impossible to establish since the only evidence of that object, or indeed of the external world itself, was no more than an amalgam of internal images.
Complicating matters for the supporters of reason over sense was the tricky matter of accepting that human frailty might lead to sensory imperfections (blindness, deafness etc) which could lead one to imperfectly use reason based on faulty sensory input. Hume suggested that a healthy skepticism that he termed "mitigated skepticism" would be a viable midpoint between too much skepticism, which would lead to rejection of all knowledge as insufficiently grounded in reality, and a dearth, which would lead to an acceptance of all knowledge as valid. An Essay Concerning Human Understanding represents his own use of reason to oddly enough undermine traditional concepts of reason itself to show a clear and convincing link between the validity of the senses to accept external stimuli such that man could rightfully draw logical and founded conclusions from their intersection.
Hume first published his ideas concerning sensory impressions in A Treatise of Human Nature (1739). Written when Hume was not yet twenty-five, this book was a powerful indictment of the overarching reliance on reason to fathom all of Nature's secrets. Unfortunately, the buying public did not agree and sales flat lined, causing him to re-write it. This revised version, which Hume called An Essay Concerning Human Understanding, was artistically superior and sold well. He divided his book into twelve sections, summarized thus:
(I) Of the Different Species of Philosophy: Hume's opening section introduces philosophy as having two broad parts: moral philosophy or the science of human nature and natural philosophy.
(II) Of the Origin of Ideas: In the ongoing debate over which is the more potent, immediate sensory impressions of objects or abstractions of thought recalled in memory, Hume declares that the former overrule the latter. He also suggests that the reliability of cause and effect is questionable since past performance is no guarantee of future performance.
(III) Of the Association of Ideas: Ideas tend to combine in predictable sequences: those that resemble each other; those that are contiguous to each other; and those that retain a cause and effect on each other.
(IV) Skeptical Doubts Concerning the Operations of the Understanding: Hume emphasizes the need to retain a healthy sense of skepticism, not too much and not too little. Sensory impressions, as always, take precedence over reasoned thought: "The most lively thought is still inferior to the dullest sensation."
(V) Skeptical Solution of these Doubts: Human beings tend to assume that habit or custom is a sufficiently valid reason to continue an action. Not so, warns Hume.
(VI) Of Probability: Hume defines "probability" and "chance" as terms that incorrectly influence human beings as to the most likely outcome of an event. Probability and chance are no more effective at prediction than habit or custom.
(VII) Of the Idea of Necessary Connection: If custom, habit, probability, and chance do not effectively predict events, then what does? Hume suggests that "the observation of constant conjunction of certain impressions across many instances" will prove so.
(VIII) Of Liberty and Necessity: Hume notes that if one follows cause and effect to its ultimate cause (God), then one as an ultimate rationalist might refuse to accept God as the Primum Mobile of all things.
(IX) Of the Reason of Animals: Animals learn in ways not unlike how people do. Constant experience with related stimuli seems to teach better than reason--which animals lack.
(X) Of Miracles: Hume notes that miracles are witnessed only by country bumpkin types and are thus unreliable. The church took extreme exception with this section.
(XI) Of a Particular Providence: Though we might like to think that God exists, we have no experience to infer either His divine existence or an afterlife.
(XII) Of the Academic or Skeptical Philosophy: In the first part to this section, Hume lists competing variations of skeptical argument. In the second, emphasizes the need to eliminate the sole reliance on cold reason and logic to solve the assorted ills of mankind.
David Hume tried mightily to undermine reason as the sole factor in human discourse. Ironically, he was so successful in doing so that his books helped to usher in the next generation of Romantics who assumed much the same.