I would not say that ethical behavior is not possible for the atheist or agnostic. It is. A couple of pretty good examples are Bertrand Russell and Jean-Paul Sartre. However, I will have to say that if we take the human lot as a whole, these two men must be seen as exceptions.
Huston SmithThe Sufis say there are three ways to know fire - by hearing it described, by seeing it, or by being burned.
Huston Smith