enkidu is just believes in the natural ontological argument when it comes to epistemology. He doesn't allow that moral knowledge can come from other sources and the possibility that moral facts does not exist at all. How is moral knowledge possible then? Enkidu would have you believe there is only one valid way,what he simplifies as "appeal to science". However there are a lot of possibilities and here are a few more clusters of arguments in moral epistemology. The sociological argument, the psychological argument, the ontological argument, and the evolutionary argument.
The sociological argument of how we get moral knowledge is one two choices: No moral facts exist to be known, since moral disagreements exemplify clashes in moral sensibility rather than differences about matters of fact. However you can admit that moral knowledge does exists, but moral facts are relative to the social group in which moral sensibility is formed with the result that no moral truths are known to hold universally.
Psychological argument: This is best argued by David Hume who suggested that moral knowledge is not even possible based on the fact that morals motivate us to act. If morals are based on reason or knowledge so that they consist in true or false ideas, they would have to be in themselves incapable of having this direct influence on our actions (Hume, Treatise, Book III, Part I, Section I, Paragraph 6.) As he famously said, it is not contrary to reason to prefer the destruction of the whole world to the scratching of a finger. The argument is expressed as a valid deductive argument from three premises: (1) If moral knowledge is possible, then some moral judgments are beliefs. (2) Our moral judgments by themselves necessarily give us some motivation to act, even without the accompaniment of already existing desires. (3) A belief by itself, unaided by already existing desires, can never give us any motivation to act. Therefore, moral judgments are not beliefs. Therefore, moral knowledge is impossible.
Ontological argument: Here is where Enkidu really gets selective. There are three possibilities where we get moral knowledge through the ontological argument. Theological, non-natural, and natural. The theological position is that moral knowledge is basically the will of God/s. The non-natural ontological argument is the proposition that moral knowledge has its basis in non-natural aspects of the world that can be apprehended only through a faculty of moral intuition or reason that is independent of sense experience. Moral reality is reducible to neither the natural nor the supernatural and requires a mode of apprehension comparable to mathematical intuition. The natural ontological argument as advocated by Enkidu argues that moral knowledge should not be more problematic than other kinds of knowledge of the natural world and is empirically observable.
Evolutionary moral epistemology: The Darwinian argument that morals are about survival and reproduction and have nothing to do with moral truth. In addition while the intuitive, emotional basis of moral judgments was useful to our ancestors, this basis is out-dated and unreliable in modern industrial society and thus current moral thought in such society.
http://plato.stanford.edu/entries/moral-epistemology/ 
