@Poseidon,
Good is not the opposite of evil, certainly not the way of cold/hot or big/small. Consequently, it's absurd to think of them as necessarily counterbalancing one another.
Good and evil are both the opposite of
nothing.
We have a moral baseline of
non-judgement. If we witness someone tying his shoe, our moral barometer is not even triggered. We don't care; we forget about it.
If we witness someone holding a door for someone, it triggers a very mild judgement of
good. If we witness someone absentmindedly letting a door slam in someone's face, it triggers a mild judgement of
bad. These are in contrast to the absence of judgement.
And if we witness someone saving a life, or if we witness someone committing a cold-blooded act of murder, we have much more extreme moral judgements.
One need not have ever known evil to understand good. And one need not have ever experienced good to understand evil. All we need to understand is how divergent such extremes are from normal, neutral experience.