Did you know Immanuel Kant philosophized on hell? Well, if you want to be factual then it's correct to say he didn't philosophize on hell, but I think you can use his moral philosophy for a philosophical defense of hell. I haven't studied that much, outside of theology that is, on the concept of hell and was wondering if one could develop a philosophy for the need of a place called hell and I actually think Kant's moral argument fits nicely in what one could call a defense for hell. Not familiar with Kant's moral argument? Simplified version: What would have to be true for ethics to be meaningful? There would have to be a god of some kind, one who is just and powerful enough to make the wicked pay for their crimes, perfect in his understanding of the evidence, and he must be righteous and incorruptible. Omnipotent, omniscient, righteous, and holy (sounds like the God of Christian theism, but I won't go there).
From his Critique of Practical Reason:
1. Happiness is what all human beings desire.
2. Morality is the duty of all human beings.
3. The unity of happiness and duty is the greatest good.
4. The greatest good ought to be sought.
5. But the unity of desire and duty (which is the greatest good) is not possible by finite human beings in limited time.
6. And the moral necessity of doing something implies the possibility of doing it (ought implies can).
7. Therefore, it is morally (practically) necessary to postulate: (a) a Deity to make this unity possible (i.e., a power to bring them together), and (b) immortality to make this unity achievable.
Kant points out in his book that we not only exist phenomenally, but also noumenally so morality demands an afterlife and a meeting with the omniscient, omnipotent, holy, and eternal Judge of everything.
How can this philosophy apply to hell? Whether it's annihilation or eternal torment, I think both can apply to Kant's moral philosophy. Our actions in this world (phenomenal) affect our future in the afterlife (noumenal) which said actions will be judged by the one who alone can judge correctly. As Kant said in his moral works, morality only makes sense if there is an afterlife. Why? Because fallible, finite human beings cannot achieve the unity of desire and duty in a limited amount of time. That unification takes an eternity to achieve.
Wouldn't this philosophy imply universalism and not a hell? Not necessarily. Those who lived a life in pursuit of the greatest good then would be rewarded with immortality in a unified bliss of desire and duty, achieving the "holy will" or "perfection", while those who did not live their life in pursuit of the greatest good would be judged and sentenced to hell (whether hell is annihilation or eternal punishment). What about those persons who lived a mixed life? Well, understand the Judge is omniscient (all-knowing) and will judge correctly.
I didn't aim for the bullseye in this post, understand I'm still working this out and blogging my thoughts helps out.