Leibniz's argument for the notion of the best of all conceivable worlds, usually known as Leibnizian optimism, is provided in its entirety in his book Theodicee (1710; Theodicy), which was devoted to justifying God's justice (see theodicy). God is all-powerful, all-knowing, and all-benevolent; 2. everything that has a beginning must have an end 3. God could have prevented all evil, but chose not to 4. Therefore, there must be a good reason for why God allows evil to exist. In modern terms, Leibniz is saying that although life on Earth as we know it is bad, maybe life elsewhere is better so therefore it's okay if things here aren't perfect. This idea that life is fine even though it's not perfectly fair or good isn't new. Ancient philosophers such as Plato and Aristotle said similar things about reality being "good enough" so people should just get used to it. Modern scientists have also suggested that life on Earth is pretty much paradise compared with some of the other places in the universe where conditions are very different. For example, one study has estimated that living outside Earth's atmosphere would be dangerous for any organism that can't protect itself from cosmic rays. But this doesn't change what you feel when you look around at all the suffering in the world.