Why do people need religious beliefs and ideas?
I think that most people who do just adhere to religious beliefs still believe in the existence of evil but they do not see it in exactly the same way, as the devil, but real, nevertheless. It does seem that most secular philosophers still see a basic duality between good and evil. Mill saw human beings as having higher and lower pleasures. T Huxley, who was an agnostic saw conflict and destructiveness as problems.
In the psychoanalytic perspective saw the tension between Eros and Thanatos. These were the life and death forces, but, to some extent to some extent his whole notion of the death instinct seems a bit like evil because it is about destructiveness. He sees life as being about instincts, with the superego being like conscience, with ego mediating between the two.
Perhaps complexes are like inner demons.
Jung certainly saw complexes as being like demons arising from the unconscious. Of course, I am aware that whether to rank Jung as falling into the religious or secular is highly debatable in itself because he wrote so much about religion and evil, and in some places he writes as if he is outside of religion and critical of it, but, at times, he writes as though he is coming from a religious perspective, as expressed in his famous interview quote, 'I don't believe in God, I know.' In some ways, he seems to come from a Gnostic perspective. He was involved in a lot of dialogue with a theologian, Victor White, about the whole problem of evil. Essentially, he seemed to side with the view of evil as a real force, as in contrast to one Christian perspective that evil is simply the absence of good. This was all documented by White in a book, 'God and the Unconscious.'
However, my main point is that it is not necessary to believe in God in order to believe in the existence of evil. That is not to say that all philosophers in the secular tradition necessarily believe in the existence of evil as a force. It is an area worth researching. I would be surprised if many believe in literal demons. Probably a lot of esoteric Eastern philosophers do believe in them. I know that many in the theosophical tradition do.
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