I will introduce diffeology as a natural generalization of differential geometry, specifically adapted to singular and infinite-dimensional spaces. I will then show how it resolves a long-standing problem in theoretical physics: establishing a direct geometric bridge between Feynman's path integral intuition and the Dirac–Souriau geometric quantization program, through the construction of the prequantum groupoid. From this perspective, quantisation turns out to be simply the intrinsic geometry of the fluctuations in the classical space of motions.