Did not write about anything, but his teachings were passed down through the works of his students, most notably Plato, who immortalized Socrates' dialogues and ideas in his writings. Declared a distinction between true knowledge (eternal, absolute, and unchanging) and opinion (temporal, changing, and relative). "Know thyself" - "An unexamined life is not worth living." Socrates believed that one's death is not the end, but merely a departure for afterlife. And that death is the departure of the soul for the eternal world.