Our picture of Socrates derives mainly from several short, inconclusive dialogues by his disciple Plato. In these, Socrates, believing that "the unexamined life is not worth living," typically challenges the conventional beliefs of his fellows, both ordinary people and more sophisticated thinkers, with questions about how human life should be lived. When his inter-locutors prove unable to defend their opinions on such questions, Socrates offers his own, radical, positive agenda in their place. We are happy, he thought, when our souls are in the best condition—when, as he believed, we have the virtues of character, especially justice. Since we all want to be happy, we will inevitably do what is virtuous if we know what it is. Hence happiness is achieved by removing ignorance and vice from our souls and replacing them with knowledge and virtue. Socrates' moral seriousness and courage, in discussion and in life, won favor not only with posterity but also with many of his contemporaries—but not all of them: in 399 he was tried and convicted on a charge of impiety, and put to death.