Perhaps surprisingly, nominalistic scruples were not the driving force behind the development the Calculus of Individuals. Instead, their goal is to solve a technical problem in Carnap’s Aufbau (1928) (see  section 4 below),  and to this end they employ both set-theoretic and mereological notions. Leonard, in his Ph.D.  thesis (supervised by Alfred North Whitehead), presents his calculus as “an interpolation in Whitehead and Russell’s Principia Mathematica between *14 and *20” (Leonard 1967), and makes liberal use of class-terms in the formulation (Leonard 1930). Also the joint paper of Leonard and Goodman is formulated using class terms, as is the system Goodman used in his own Ph.D. thesis, A Study of Qualities (1941, SQ). It is not until his joint article with Quine (Goodman and Quine 1947) and his Structure of Appearance (1951, SA) that Goodman eschews the use of set theory to formulate the Calculus of Individuals.