This chapter focuses on Husserl’s Ideas I (1913) and the way it manifests what has been called the “Göttingen idea of preestablished harmony between mathematics and physics.” Whereas the previous chapter discussed the notion of definiteness as Husserl’s clarification of the guiding ideal of pure mathematics, this chapter examines the same notion as a guide for the applied sciences and especially for physics. It shows how the mathematical ideal serves as an ideal of rationality, as a device that guides our empirical investigations “regulatively.”