Full Idea
That the straight line between two points is the shortest is a synthetic proposition. For my concept of the straight contains nothing of quantity, but only a quality. The concept of shortest is additional, and cannot be extracted by analysis.
Gist of Idea
That a straight line is the shortest is synthetic, as straight does not imply any quantity
Source
Immanuel Kant (Critique of Pure Reason [1781], B016)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.145
A Reaction
We should ask Kant to define 'straight' without mentioning 'shortest'. If you think of a long walk between two towns, it becomes obvious that the straight line will be defined by being the shortest line.