René Descartes spent most of his childhood in solitude, a situation that also came to characterize his adult life. Fortunately, these countless lonely hours helped Descartes produce the declaration that changed all philosophy: “I think, therefore I am.” Eventually convincing himself to doubt and disregard sensory knowledge, Descartes found he could prove his existence through his thoughts. This internal information, he believed, was the true...

The "Long Debate" on the nature of truth, the scale of real values, the life one should aspire to live, the character of justice, the sources of law, and the terms of civic and political life is encompassed by the name philosophy. Three persistent themes--understood as problems--are knowledge, conduct, and governance, on which there is a storehouse of insights, some so utterly persuasive as to have shaped thought itself. Beginning with Plato and...