Nearly six hundred years ago, a short, genial, cannily alert man in his late thirties took a very old manuscript off a library shelf, saw with excitement what he had discovered, and ordered that it be copied. That book was the last surviving manuscript of an ancient Roman philosophical epic, On the Nature of Things, by Lucretius--a beautiful poem of the most dangerous ideas: that the universe functioned without the aid of gods, that religious fear was damaging to human life, and that matter was made up of very small particles in eternal motion, colliding and swerving in new...