In this chapter, I will discuss Formal and Transcendental Logic (FTL) (1929) as a detailed example of Husserl’s usage of radical Besinnung. I will start by sketching an overview of Husserl’s involvement with mathematicians in the 1920s. At that time Husserl lived in Freiburg, where he did not have constant contact with mathematicians the way he had previously had in Göttingen. His most important sources seem to have been his former student and assistant Oskar Becker (1889–1964), some other students and followers, and the publications he had in his private library. Three books in mathematics published in the 1920s, and held in Husserl’s private library, contain markings that indicate that they were studied rather carefully (see Hartimo 2018c). These works, one by Hilbert and two by Weyl, will be discussed in detail in the first section of this chapter.