Full Idea
If I know that the earth is a sphere, and its surface the surface of a sphere, then from a small part of the latter I can know the diameter, and hence the complete boundary, and in accordance with a priori principles.
Gist of Idea
If I know the earth is a sphere, and I am on it, I can work out its area from a small part
Source
Immanuel Kant (Critique of Pure Reason [1781], B787/A757)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.653
A Reaction
A nice example, though it may be optimistic in its assumption that you can know you are on a sphere rather than an egg-shape. I agree with Kant, but speculative metaphysics should always be accompanied by humility and health warnings.