 Chevalley went on to single out the discovery of a continuous nowhere differentiable function, due to Weierstrass, as the most important element of this revolution. As Weierstrass’ function can be given in terms of a Fourier expansion with a quite normal appearance, it became obvious that many demonstrations in mathematics assumed closure condition that needed to be rigorously established. The concept of limit, as defined by Weierstrass, was the powerful tool that allowed such investigations. The reconstruction of analysis pursued by Weierstrass and his followers turned out to be not only foundationally successful but also mathematically fruitful. Here is how close Chevalley comes to characterizing this style: