 In the subtle frontier between logic and physics, Décio Krause developed an impressive work related to the quantum challenges to the notions of identity and individuality. Already in his PhD dissertation, under the supervision of Newton da Costa, Krause supplied a semantics to the so-called “Schrödinger logic” (da Costa and Krause 1994, 1997) designed to account for elementary quantum particles. This work led him to formulate his quasi-set theory (Krause 1991, 1992, 1996a, 1996b), according to which there are urelements for which identity makes no sense. Krause’s semi-extensional quasi-set theory was compared with the intensional quaset theory developed by Maria Luisa dalla Chiara and Giuliano Toraldo di Francia (dalla Chiara, Giuntini, and Krause 1998), and was applied to particular problems in the foundations of quantum mechanics (Sant’Anna and Krause 1997; Krause, Sant’Anna, and Volkov 1999; Krause 2000; Domenech, Holik, and Krause 2008).