Great advice RC! Really understanding the mindset of the antagonists is key.
Here's an example of how I, as a DM, set up philosophical debate in a campaign:
1) I knew what the PCs believed, what their goals were, how far they were willing to go for those goals, what they cherished, who they served, etc.
Rainfall (PC) is trying to find out who had slandered her family's name and sabotaged her father's career and sent him into imprisonment. A child of nobility she believes she never has to explain herself to others. She is willing to kill the guilty and has teamed up with the half-fiend Telakil in order to escape mysterious assassins; the two have become lovers.
2) I focused on the NPCs the players were most interested in and spent time fleshing out their motives, their web of relationships, what higher power(s) they believe in, etc.
Telakil (NPC) is torn between his evil and good natures, knowing full well that he is leading Rainfall and her allies into a trap orchestrated by his vile father, yet he finds himself falling in love with Rainfall. Moreover, his dreams are filled with images of a benign Goddess, and the possibility of faith has been born in his heart. He has been an assassin most of his life, is willing to kill innocents...though he is beginning to question himself.
3) Based on this information, I think of a dilemma that would put the PC and NPC in some sort of conflict that requires thought and discussion before action.
What if Rainfall and her companions come across a demon with information about the conspiracy to bring down Rainfall's noble house? Let's up the ante, and have the demon hint that it knows where her father is being imprisoned. The demon offers to help them to the best of its abilities, but requests that they help redeem its soul. Telakil doesn't believe the demon; in fact he doubts whether a "fallen angel" can ever be redeemed - its choice is final in his mind. Now the stage is set for a question: Can a demon be redeemed?
4) Encourage players to think and discuss. Do this by modelling philosophical inquiry with NPCs, not forcing the issue on the players, and providing "carrots" (XP, favors owed by NPCs, bonus modifiers down the road, an extra feat like "reconciliation of ethics" or what have you).
It begins when the demon reveals that Telakil plans treachery. Backed into a corner by the hostile PCs, Telakil states his belief that demons can't be redeemed, and sneers at their "guilibility" before fleeing. The demon says it had seen the light of redemption in Telakil's eyes and so hoped to claim that light for itself. If they accept the demon's aide they should indeed get the information about Rainfall's father (the carrot). They could also get extra XP for role-playing, or even the demon's eternal gratitude if they manage to redeem it.