Giorgio Lando

Mereologydis (mereology as a discipline) is simply the study of the relation of parthood and of strictly related topics.

Mereologytheo (mereology as a theory) is a theory that characterizes parthood and other connected relations (such as composition) in a certain way.

These axioms imply some theorems: these theorems are the content of a certain mereologytheo.

Classical Extensional Mereology, and has been formulated—in different forms, with different purposes, and in different contexts—by logicians and philosophers such as Leśniewski, Tarski, and Goodman.

Its theorems do not establish what is in the domain of their quantifiers.

mereologyphi (mereology as a philosophical thesis) is the philosophical thesis or contention that Classical Extensional Mereology (the mereologytheo par excellence) is the general and exhaustive theory of parthood and composition

It is the philosophical thesis that, in a large and significant domain of things, parthood is transitive, and that, given some things, there is one and only one thing composed of them.

In the most radical (perhaps too radical, as we will see) version of mereologyphi, this large and significant domain is actually the unrestricted domain: the level of generality and exhaustiveness of the theory is taken to be maximum.

Mereologyphi has also been dubbed mereological monism by Kit Fine

First, there is no interesting sense in which mereology is a logical doctrine.