Gives the character knowledge of whether a circumstance or action is right or wrong, offering clarity in situations where the character may be suffering from a dilemma.
The correct answer in these cases should be framed so as to offer the best possibility for the character's survival without causing ill will towards others. Inherent within the knowledge should be elements of character restraint, generosity, fortitude and patience . . . the character should not insist that the forthcoming answer require no effort or that it should provide instant gratification.
Balancing this, should the character pursue the course suggested (and the DM must propose this knowledge as a guideline and not an absolute list of required actions), the DM should be careful to balance moments in the campaign in the character's favor, allowing the character to avoid saving throws or gratuitous attacks from monsters who could reasonably choose to parley instead. Naturally, should the character behave immodestly in situations, such dangers should increase, indicative of the character 'falling by the wayside.'
Remember that the character must deliberately choose to be introspective in order to determine a moral answer; the player is also free, once the answer is given, to take whatever action they like. The DM must be prepared to give an answer that in no way serves the campaign or the player - but rather, the answer should reflect what would truly be the least selfish approach to the problem the character could adopt.
It is suggested that if the DM feels that this is beyond their ability to give, it is suggested that the players as a group decide the answer, and that the DM then 'adjust' the campaign to empower that answer. Alternately, the DM could simply discontinue the use of this ability.
I would imagine that many DMs would pale and shudder at the idea of this sort of thing - particularly at the suggestion that the campaign itself should be moderated in favor of the player choosing to follow through.
I'm not certain everyone understands this, so I will wax a bit upon it.
Suppose the players find themselves trapped between two groups who appear to both possess a blood-thisty approach to life, seem equally difficult to approach and which pose an equal threat to the party. Not knowing which side to trust, the character with the Define Morality ability considers, silently, the situation, going over all the details learned to that point, in their mind studying the faces and the words heard from both groups.
What answer should the DM give? Particularly if both sides ARE of the same nature?
Fundamentally, I presume the DM has greater knowledge than the player, enabling a superior judgement call on what would be the correct side to trust. Furthermore, if the situation is well-designed, there should be good cause to believe that the appearance of the two forces is a veneer intended to hide the true nature of at least one force if not both. Finally, I propose that the DM should manufacture an inherent good that allows the player to know which side would, in fact, protect the player and party from the other.
At that point, the DM should say, "You can better trust Group B" - adding, to place pressure upon the player, elements into the action that would require the character to sacrifice, make promises or pledges, wait until the time is right and so on.
The player would then be free to ignore this advice, raise swords and perhaps succeed against either group - because the moral path should not be the only path! The player should not be punished for not deciding to approach the matter considerately.
However, the player should be meaningfully rewarded if choosing to follow the advice given by Define Morality - even if that advice does, in the short run, result in the character suffering somewhat. The moral path is not always the easiest! It is, however, the one that should bring the greatest reward, even if the long run is required to identify the full fruition of that reward.
I hope that helps define the ability. I would hope that a DM here or there would consider the value something like this could add to the campaign. At the same time, I also recognize how truly difficult it would be to either manage the answer once it was given or encourage players to accept it as truth.