Applied mathematicians often work with circles, but I'm guessing it's an abstraction that cannot save all the empirical data. Can we conceive of a perfect circle in our visual field -- as apparently they do not exist in nature?
Zombies in philosophy are imaginary creatures... Can we really imagine zombies? Daniel Dennett thinks those who accept the conceivability of zombies have failed to imagine them thoroughly enough
And Chalmers p153 says
it is arguable that one can modally imagine S when S involves an a priori contradiction. An example may be a case in which one imagines a geometric object with contradictory properties. In cases like this, one imagines a situation in something less than full detail... S is positively conceivable when it is coherently modally imaginable
So it seems being imaginary is usually a weaker state than being conceivable: we can imagine incoherent things perhaps like zombies, just not fully.
But with visible things appearing to us, that may not be the case. Because it also seems that we can conceive of perfect circles, but not imagine them -- not conceive of what it would be like to see a perfect circle.
If so, is that what makes a "circle" a phenomenological "essence" in Husserl's sense? i.e. is anything we can conceive of but not conceive of appearing to us an "essence"?