[b3kart]

No human has ever set eyes on a perfect circle, because it (most likely) doesn't exist in nature. As such, I would argue that a perfect circle is a concept (or a model, if you will) that we've invented to make it easier for us to deal with an imperfect real world. I would not call identifying properties of such a model an act of discovery: one can come up with any model, no matter how far from reality, and use some set of axioms to identify its properties, but none of it will make said model real or fundamental in any way. The best we can hope for is that the model will be useful for making predictions about the real world.