 Opus maius, Part four, deals with mathematics and the applications of mathematics. Bacon presents reasons for a reduction of logic to mathematics (a kind of reversal of modern logicism) and sees mathematics as the key to an understanding of nature (Perler 2005). Clearly, he is proclaiming the “usefulness” of mathematics for knowledge; he is not doing mathematical theory. And the branch of mathematics that is important here is geometry. He applies it in detail in the optical works edited by Lindberg. Following his abbreviation of the De multiplicatione specierum in part four of the Great Work, which shows how mathematics might be applied to physics, he deals with the application of astronomy/astrology to human affairs, the uses of mathematics in religious rites as in chronology, music, symbolism, calendar reform, and geographical knowledge, and a resume of astrology (Rutkin, 2019). It used to be thought that Bacon was a Platonist in his view of the absolute priority of mathematics. More recently, that view has been seriously qualified. He does not reduce physics to mathematics. Indeed, his explicit work on mathematics, the Communia mathematica, is not an exercise in mathematics, but a presentation of the “common notions” that are important for a variety of mathematical practices. Bacon himself acknowledges those who were better mathematicians, namely, John of London, Pierre de Maricourt, and Campanus of Novara (Molland 1997). In general, Bacon is more interested in how mathematics can contribute to knowledge of the world as an aid to missionary activity. He sent a map of the world to the Pope. 