Are babies born knowing what a circle is?
This is open to empirical investigation. here's some background:
http://www.psychologytoday.com/blog/babies-do-the-math/201101/brainy-babies
"babies appear to be born knowing that objects cannot magically appear or disappear, that they cannot pass through each other, and that they cannot move unless contacted by another object. These expectations hold for objects, but not for non-object entities like substances (e.g., liquid, sand)."
So babies are born knowing quite a lot of physics

.
"by their first birthday (and long before they can talk), babies exhibit quite sophisticated number knowledge. They can enumerate visual and auditory items, items presented sequentially and items presented simultaneously."
Nothing there about a circle being "in there". "Numbers" and "assuming continuing existence" are in there because they are useful to evolution. But is spotting a circle useful to evolution? Being able to choose two apples, rather than one, is obviously useful; but taking the more circular apple doesn't seem useful. My guess is that the circle was invented, and has to be explained to children - it's not "in there", and no perfect circle is "out there", so it had to be invented.