Reason--According to David Hume (1711-1776)

The use of the philosophical of word ‘substance’ derives—I think—frrom the Latin word that Cicero used to translate Aristotle’s term for ‘being’ or, morely accurately, ‘what-it-is-to-be.’ Anyway, Hume took a very tough line on that word which had been so thoroughly--and of course intentionally--emptied of all possible sensory content as to become just the sort of empty and therefore,he thought, meaningless metaphysical abstraction he despised. Like certain 20th century philosophers, Hume had a theory of meaning that excluded words or concepts which, by definition, could not be tested or verified. (For that and other reasons which I’m about to get to, Hume was the first modern philosopher—or, at any rate more modern than Spinoza, who in turn was more modern than Decartes etc.) I’m inclined think that that theory of meaning followed, more or less logically, from the great idea that must have occurred to him when he was a very young man (the Treatise of Human Nature was published in 1738-9, while he was still in his twenties): Hobbes was right, reason is merely reasoning from premises to conclusions which is all that it can be if there is no God or if God=Nature; Right Reason therefore is nothing more than common-sense. And if reason is reasoning, it follows—this was Hume’s great idea—that neither matters of fact nor morality can ever be established by reason alone i.e. by reasoning a priori—as he says in the Treatise, statements of fact do not in themselves logically imply statements of moral principle or obligation. Hume condensed the difficult arguments of the Treatise in the shorter, wittier, more commonsensical Inquiry Concerning Human Understanding, published about ten years later.
A commonplace of intellectual history is the notion that the 18th century was an age of reason. It would be more accurate to say that that century was an age of intellectual warfare, which continues, over the meaning of the word ‘reason.’ Hume’s was the first “critique of pure reason”—written in the most reasonable sounding voice you ever heard. The history of philosophy since Hume is in part a history of attempts to evade his conclusions. Kant’s Critique of Pure Reason is the greatest of these.