Complex sentence: He furthered the idea by suggesting that evil is necessary in a perfect world, as it allows good to be expressed and appreciated and suggested that, if there was no suffering in the world, freedom from suffering could not be truly appreciated.
Split into simpler sentences: He furthered the idea by suggesting that evil is necessary in a perfect world, as it allows good to be expressed and appreciated. He suggested that, if there were no suffering in the world, freedom from suffering could not be truly appreciated.