All in Evil

How do we recognize things we have never known? We know love when we see it, because we have felt it, and have watched it in action. We know good things when we see them too because we taste them, feel them, and witness the mercy and grace that they exude, but how is it that we have come to recognize evil? Where did our understanding of evil come from? I would like to think we know it because we have seen it in the world, being perpetuated by the unrighteous, but scripture tells us it comes from our heart... it wells up from within us.