Well, I am not going to interject my beliefs on all you GFY'ers but one thing that cannot be proven wrong is that you are in control of your own actions.
Whether you believe in a god, a monkey named Charlie Bean, or nothing at all makes no difference. If you were intelligent, your actions would be the same even if you could prove or disprove the existence of god.
At the end of the day, being "Good" has more rewards than being "Evil". In the long run, if you help society, society eventually helps you.
