Lucretius' poem On the Nature of the Universe combines a scientific and philosophical treatise with some of the greatest poetry ever written. With intense moral fervour he demonstrates to humanity that in death there is nothing to fear since the soul is mortal, and the world and everything in it is governed by the mechanical laws of nature and not by gods; and that by believing this men can live in peace of mind and happiness. He bases this on the atomic theory expounded by the Greek philosopher Epicurus, and continues with an examination of sensation, sex, cosmology, meteorology, and geology, all of these subjects made more attractive by the poetry with which he illustrates them.