Because my definition of good includes God , who is objective on the subject.
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God makes extremely rare personal appearances, and when he does, he does not hold question-answer sessions. I fail to see how you could define good unless God were a lot more forthcoming.
Socrates was not much of a believer, and one of his main topics was the definition of "the Good". He did not seem to require a Deity of Deities to reach a definition. Mostly, he used logic. Atheists today do the same thing. The main problem atheists seem to have today is that when atheists are called all sorts of names, they do not join one another in rebutting what is said about them.