An opinion that something is universally true clearly goes further than experience can warrant. An opinion that something is necessarily true (that is, not merely is true in the existing state of things, but would be true in every state of things) equally goes further than experience will warrant.
Kant proceeds to reason as follows:
But the dry-rot of reason in the seminaries has gone to the point where such stuff is held to be admirable argumentation.
- Geometrical propositions are held to be universally true.
- Hence, they are not given by experience.
- Consequently, it must be owing to an inward necessity of man's nature that he sees everything in space.
- Ergo, the sum of the angles of a triangle will be equal to two right angles for all the objects of our vision.