Good and Evil in a Modern World

One notion that this liberated postmodern man has been unable to let go of, despite having recently freed himself of the religious notions of his parents and rebuilt his worldview entirely on only what can be proven scientifically, is the notion of good and evil. He can't seem to let it go. It must exist, and it must be explainable within his new peridyme, because without it there is no meaning or passion or texture to life. In fact life becomes a story without a plot. Everything is flatlined. I mean, think about it. Why do we do anything, really. It's usually because we think it is a good thing to do. Even when we do something that we know is wrong, we have somehow justified it in our minds before we do it. It is the basis of every argument, every endevour, every motivation. We wake up to it in the morning, and we fall asleep to it every night (unless, of course, it's keeping us awake). God might be dead, but good and evil are alive and well. How then do we explain what it is? I mean, this used to be easy. Good is good because God said it is good. But now that God is out of the picture, what justifies our indignacy at being wronged. Some have said that good is what keeps the human race going. It's just another word for efficient cohabitation. Others have said good is what makes us happy, and evil is what causes suffering. But if we reduce good and evil to a theory, what do we have left to evaluate our theory with. We can't really evaluate if the longevity of the human race is good or that suffering is bad, because these evaluations are made outside of our definition. Even these two theories themselves are incompatible, as the longevity of the human race seems to bring about more suffering than it does happiness. The more we think about good and evil, the more we come to realize that they really stand outside of everything. Despite not being able to agree entirely on what is good and what is evil, we all seem to know they exist and are worth defending, even though we don't really know why.