Semantic challenges to divine command theory have been proposed; the philosopher William Wainwright argued that to be commanded by God and to be morally obligatory do not have an identical meaning, which he believed would make defining obligation difficult. He also contended that, as knowledge of God is required for morality by divine command theory, atheists and agnostics could not be moral; he saw this as a weakness of the theory. Others have challenged the theory on modal grounds by arguing that, even if God's command and morality correlate in this world, they may not do so in other possible worlds. In addition, the Euthyphro dilemma, first proposed by Plato (in the context of polytheistic Greek religion), presented a dilemma which threatened either to result in the moral arbitrariness of morality itself, or to result in the irrelevance of God to morality. Divine command theory has also been criticised for its apparent incompatibility with the omnibenevolence of God, moral autonomy and religious pluralism, although some scholars have attempted to defend the theory from these challenges.