Full Idea
For Kant, geometry studies the forms of perception in the sense that it describes the infinite space that conditions perceived objects. This Euclidean space provides the forms of perception, or, in Kantian terms, the a priori form of empirical intuition.
Gist of Idea
Geometry studies the Euclidean space that dictates how we perceive things
Source
report of Immanuel Kant (Critique of Pure Reason [1781]) by Stewart Shapiro - Thinking About Mathematics 4.2
Book Reference
Shapiro,Stewart: 'Thinking About Mathematics' [OUP 2000], p.88
A Reaction
We shouldn't assume that the discovery of new geometries nullifies this view. We evolved in small areas of space, where it is pretty much Euclidean. We don't perceive the curvature of space.