One problem raised by Euthyphro is, of course, the question of why, if "good" is determinable independently of the gods, we would need the gods to tell us anything. If moral laws are rational, then we should be able to figure them out by using our reason; so why do we need divine revelation? Saadia Gaon, the first major systematic Jewish philosopher (post-Philo, at least), suggests that reason just isn't precise enough; it tells us what things are right and wrong, but doesn't give us the details regarding how to act rightly. That's where revelation comes in. Here is a particularly interesting example:
Whereas reason regards fornication as reprehensible, it does not define how a woman is to be acquired by a man in order to be considered as belonging to him. [It does not state, for example,] whether that is to be effected by means of a word only, or by means of money only, or with her consent and the consent of her parents only, or by the testimony of two or ten witnesses, or by having all the inhabitants of the town bear witness thereunto, or by marking her with a sign or branding her.So one of the options is "her consent" while another option is "branding her." Apparently, reason alone doesn't tell us whether women are human beings or cattle. I find this a bit disturbing; if reason can't figure that out, I'm not sure about reason's prospects for anything else!