Mereology, is a part-whole theory, also called the theory of collective sets. It was founded in1916 by Stanislaw Lesniewski and this is an alternative theory versus the classical set theory by Georg Cantor. These two theories are usually teamed up together as Leśniewski himself was referring to the concept of the set by Cantor and Cantor is considered the "main" ideologist of the set theory. However, when analyzing the original texts of various authors, it seems that the very concept of a collective set is closer to the idea of Richard Dedekind rather than that of Georg Cantor. It is known that Cantor borrowed some concepts on the notion of set from Dedekind, whose ideas were also known to Leśniewski, however, there is no study on this topic. This work is therefore an attempt to compare some set-theoretical concepts of both of these authors, i.e. S. Leśniewski and R. Dedekind and the presentation of their convergence.