David Hume (1711-1776) is unique amongst philosophers in that, according to all accounts, he seems to have been a very pleasant and sociable person. He was born into a relatively wealthy Scottish family and was directed toward a profession in law. Hume disliked this vocation, turning instead toward philosophy. While still in his twenties, he wrote the monumental Treatise of Human Nature, which, to his surprise and disappointment, received very little attention upon its publication. He never actually held a university post, being turned down from two appointments on charges of atheism, and made a living as a man of letters, acting variously as a secretary, tutor, librarian, and historian. He lived a great deal of his life in France, where he was very popular in literary circles.
Hume's interest in philosophy extended throughout his life, and he published numerous shorter works that tried to clarify or refine the ideas expressed in the Treatise. The Enquiry Concerning Human Understanding, first published in 1748, is a significant reworking of the first book of the Treatise. In it, he builds upon the empirical philosophy of Locke and Berkeley, and attacks the metaphysical rationalism of Descartes and others.
Philosophy since Descartes had been marked by a rough distinction between empiricist and rationalist philosophy. Rationalists tended to concern themselves with metaphysical questions of the nature of substance, of God, of the soul, of matter, and so on, and usually tried to answer these questions through the exercise of pure reason. Empiricists were more interested in epistemology and determining precisely what and how we can know, arguing that experience is the only sure guide to substantial knowledge about the world.
While providing compelling arguments against the rationalist position, Hume also managed to imperil empiricist philosophy by unflinchingly following it to its logical conclusion. If our only guide to worldly knowledge comes from experience, there is very little we can safely claim to know. Perhaps the most significant effect of the skepticism expressed in the Enquiry is the impact it had upon Immanuel Kant. Kant famously remarked that reading Hume awoke him from his "dogmatic slumber" and prompted him to write the Critique of Pure Reason, which stands as one of the most significant works of philosophy ever written.
Hume is considered an important figure in the Enlightenment, which included Rousseau, Goethe, and others. Generally speaking, the Enlightenment represents a climate of intellectual optimism regarding the capacities of human reason. It was thought that sound and sensible argument could lead to the truth and happy agreement among disputing parties. To an extent, Hume is exemplary of this vein of thought. He battles ardently against metaphysics and religious dogmatism precisely because they obscure reasoned discourse, and his own writing is a model of clarity and careful reasoning. At the same time, however, his conclusions lead inevitably to a certain skepticism about the capacities of reason, and thus undermine the very spirit in which they were reached. Perhaps this contrast only brings out further the paradoxical nature of Hume's impact: on one hand, he was the most consistent empiricist philosopher, and on the other hand, he rendered empiricism impotent; on one hand, he was an exemplar of Enlightenment thought, and on the other hand, he undermined its driving principles.