Why 'should' atheists be moral?

[Mudhammam]

Is the "all-value" conferred by you or is it instrinsic to the "supernatural" in a mysterious manner that common sense just doesn't allow us to meaningfully define?

Explain what you mean by "conferred by you".

In other words, an all-encompassing sense that originates in and by your conceptions of life and purpose.

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Well, in short, "should" implies that such and such an action entails good or desirable outcomes, depending on what it is you value. In other words, it's a question of "what does it mean to be good?" to which an answer is found in one's sense and education.

Right, I would add that while people naturally pursue what they perceive to be good, they "should" pursue what is actually good, as opposed to what is only apparently so.

By what methods do you propose such distinction ought to be made?