Leibniz says the following: "It is true, or rather it is necessary, that a circle is the most capacious of isoperimetric shapes, even if no circle really exists" in the opening line of a lengthy proof he gives for God's existence. I took calculus in college, but I don't recall what exactly these terms mean. By "most capacious" I assume he means having the most space in it, but what exactly are "isoperimetric shapes"? And why is a circle the most capacious of them? Google gives me nothing!