In philosophy and mathematical logic, mereology (from the Greek μέρος, root: μερε(σ)-, “part” and the suffix -logy “study, discussion, science”) is the study of parts and the wholes they form. Whereas set theory is founded on the membership relation between a set and its elements, mereology emphasizes the meronomic relation between entities, which—from a set-theoretic perspective—is closer to the concept of inclusion between sets. Mereology has been explored in various ways as applications of predicate logic to formal ontology, in each of which mereology is an important part. Each of these fields provides their own axiomatic definition of mereology. A common element of such axiomatizations is the assumption, shared with inclusion, that the part-whole relation orders its universe, meaning that everything is a part of itself (reflexivity), that a part of a part of a whole is itself a part of that whole (transitivity), and that two distinct entities cannot each be a part of the other (antisymmetry). A variant of this axiomatization denies that anything is ever part of itself (irreflexive) while accepting transitivity, from which antisymmetry follows automatically. Although mereology is an application of mathematical logic, what could be argued to be a sort of “proto-geometry”, it has been wholly developed by logicians, ontologists, linguists, engineers, and computer scientists, especially those working in artificial intelligence. “Mereology” can also refer to formal work in General Systems Theory on system decomposition and parts, wholes and boundaries (by, e.g., Mihajlo D. Mesarovic (1970), Gabriel Kron (1963), or Maurice Jessel (see (Bowden 1989, 1998)). A hierarchical version of Gabriel Kron’s Network Tearing was published by Keith Bowden (1991), reflecting David Lewis’s ideas on Gunk. Such ideas appear in theoretical computer science and physics, often in combination with Sheaf, Topos, or Category Theory. See also the work of Steve Vickers on (parts of) specifications in Computer Science, Joseph Goguen on physical systems, and Tom Etter (1996, 1998) on Link Theory and Quantum mechanics. In computer science, the class concept of object-oriented programming lends a mereological aspect to programming not found in either imperative programs or declarative programs. Method inheritance enriches this application of mereology by providing for passing procedural information down the part-whole relation, thereby making method inheritance a naturally arising aspect of mereology.