The Calculus of Individuals in all its formulations contains principles of mereological summation and mereological fusion. Mereological summation is a binary function of individuals, so that the sum s of two individuals a and b is such that both a and b, and all their parts, are parts of s—and also all sums of parts of a and b and are parts of s. Mereological fusion is a generalization of the mereological summation. In Leonard and Goodman 1940 fusion is defined using sets: all the members of a set α are “fused” in the sense that they, and all their parts, and all the fusions amongst their parts, end up being parts of the individual that is the fusion of set α.