For Edmund Husserl the purely perceptual world possesses an unchanging, homogeneous structure. Taking issue with Husserl, this essay isolates a subregion of artificial bodies and processes called the "built world" which lacks the structural stability exhibited by the perceptual world as a whole. The paper goes on to argue that alterations within the built portion of the perceptual world during the late Middle Ages set the stage for insights essential to the "mathematization of nature." Husser's account of mathematical physics as an outgrowth of philosophy may be mistaken.