This is the question which C.S. Lewis addresses in “Man or Rabbit?”, which many consider to be his best short essay. It is published in his book God in the Dock: Essays on Theology and Ethics, and if you’ve never read it I encourage you to set aside ten minutes or so and read Lewis’ thoughts on what “good” is, and how one can live a good life. I love this quote from the last paragraph: “Morality is a mountain which we cannot climb by our own efforts; and if we could we should only perish in the ice and unbreathable air of the summit, lacking those wings with which the rest of the journey has to be accomplished.”
Here is a longer excerpt to whet your appetite:
A Christian and a non-Christian may both wish to do good to their fellow men. The one believes that men are going to live forever, that they were created by God and so built that they can find their true and lasting happiness only by being united to God, that they have gone badly off the rails, and that obedient faith in Christ is the only way back. The other believes that men are an accidental result of the blind workings of matter, that they started as mere animals and have more or less steadily improved, that they are going to live for about seventy years, that their happiness is fully attainable by good social services and political organisations, and that everything else (e.g., vivisection, birth-control, the judicial system, education) is to be judged to be “good” or “bad” simply in so far as it helps or hinders that kind of “happiness”.
Now there are quite a lot of things which these two men could agree in doing for their fellow citizens. Both would approve of efficient sewers and hospitals and a healthy diet. But sooner or later the difference of their beliefs would produce differences in their practical proposals. Both, for example, might be very keen about education: but the kinds of education they wanted people to have would obviously be very different. Again, where the Materialist would simply ask about a proposed action “Will it increase the happiness of the majority?”, the Christian might have to say, “Even if it does increase the happiness of the majority, we can’t do it. It is unjust.” And all the time, one great difference would run through their whole policy. To the Materialist things like nations, classes, civilizations must be more important than individuals, because the individuals live only seventy odd years each and the group may last for centuries. But to the Christian, individuals are more important, for they live eternally; and races, civilizations and the like, are in comparison the creatures of a day.
You can read the rest of this essay online here, or look for a copy of God in the Dock. Our local library has a copy, if you happen to live in the Cookeville area.