"Kurt Gödel maintained a profound interest in Leibniz’s mathematical and philosophical writings during the greater part of his lifetime. The earliest evidence of his study of the Gerhardt edition of Leibniz’s mathematical writings comes already from 1929 (the relevant library slip is preserved in the GN folder 5/57, 050173), and most scholars agree that even after Gödel’s ‘phenomenological turn’ to Husserl in 1959, he still adhered to a more-or-less Leibnizian approach not only in logic and the foundations of mathematics but also in metaphysics (see, e.g., Van Atten & Kennedy 2003 and Rescher 2020). The recollections of Karl Menger and the reports of Hao Wang also attest to this fact, as does the great number of preserved notes and other unpublished handwritten material from Gödel’s Nachlass. Most interesting documents in this regard are the so-called MaxPhil notebooks (Maximen Philosophie) written between 1938 and, approximately, 1955, in which we may find an increasing number of references to Leibniz’s papers as well to the secondary literature devoted to Leibniz starting from 1943–46, especially in the MaxPhil X, XI, and XIV. The 130 pages long MaxPhil XIV written over a period of almost ten years (1946–55) may be taken as a programmatic treatise which serves as direct textual evidence of Gödel’s unrealised project of developing an extensive metaphysical system based upon Leibniz’s monadology, intertwining logic, mathematics, physics, and rational theology (cf. Crocco 2013). In this talk I wish to comparatively analyse, from a historico-philosophical point of view, Gödel’s project with the similar earlier attempt of Georg Cantor to establish a system of monadological metaphysics from the 1880s. However, this immediately turns out to be quite a challenging task due to the fact that “there is no conclusive evidence, either in his published or his unpublished work, that Gödel had read, meditated upon or drawn inspiration from Cantor’s philosophical doctrines.” (Ternullo 2018) But, upon comparing the contents of Cantor’s and Gödel’s monadology, the similarities between the two theories are quite striking: both take Leibniz as their starting point, both are informed by the relevant mathematical, i.e., set-theoretical insights, both are motivated by theological concerns, etc. Furthermore, Cantor and Gödel were both rationalists, they both endorsed actual infinity in mathematics, and they both subscribed to some form of mathematical Platonism or realism (mixed with hints of idealism). Since so many coinciding views can hardly be a product of mere chance, I will seek to uncover Cantor’s and Gödel’s respective motives and possible sources of inspiration, and to study these within wider historical, philosophical, and mathematical context. In the first part of the talk, I will cover Gödel’s study of Leibniz’s papers, as well as his references to Leibnizian ideas in both his published and unpublished works. I will then turn to Cantor’s 1885 Acta Mathematica paper, underlying the philosophical and other non-mathematical aspects and motivations behind his introduction of set theory and a novel, anti-Aristotelian theory of the continuum. Finally, I will compare Cantor’s and Gödel’s attempts at developing set-theoretical monadology, showing that Gödel arrived at a theory practically identical to Cantor’s, independently of any previous insight into what Cantor published more than half a century before him."