Negative Platonism

How is it that we know that something is bad, or imperfect? What makes a bad argument such, or a bad society? It would seem that there can only be a bad if there is a good, and there can only be an imperfect if there is a perfect. But we do not seem to have any real examples of perfection. For something to be more or less beautiful, there must be some formal constitution of Beauty itself as Plato argues. But it does not appear that we need to know these Forms through their presence, as a legitimization of our own self-satisfied certainty. Rather, perhaps we know the Forms through their absence. This could be how they gain their existence from the supreme Form of the Good which is beyond being: the Forms are not present, and so "are" not, but they are what we strive after while making what is to be good. We are Eros, born of Poverty and Craft, pursuing Aphrodite whom we have not yet grasped. We notice that a law is bad through the absence, disorder, and impropriety which is the absence of a good law, and so, without knowing exactly what a good law on the topic is or having an existing good law, we press forward anyhow.