 Today natural law theory offers the most common intellectual defense for differential treatment of gays and lesbians, and as such it merits attention. The development of natural law is a long and very complicated story. A reasonable place to begin is with the dialogues of Plato, for this is where some of the central ideas are first articulated, and, significantly enough, are immediately applied to the sexual domain. For the Sophists, the human world is a realm of convention and change, rather than of unchanging moral truth. Plato, in contrast, argued that unchanging truths underpin the flux of the material world. Reality, including eternal moral truths, is a matter of phusis. Even though there is clearly a great degree of variety in conventions from one city to another (something ancient Greeks became increasingly aware of), there is still an unwritten standard, or law, that humans should live under.