Full Idea
The proposition that with two straight lines no space can be enclosed cannot be derived from the concept of straight lines and the number two. You are forced to take refuge in intuition, ..which is a pure a priori intuition of a synthetic proposition.
Gist of Idea
That two lines cannot enclose a space is an intuitive a priori synthetic proposition
Source
Immanuel Kant (Critique of Pure Reason [1781], B065/A47)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.170
A Reaction
A very nice example. If you gave a child two rods and told them to make a shape, they might quickly learn this from experience. Kant's proposal is nice, but I am not convinced. We learn that to create shapes you must turn corners.