The art of thinking · Parts, wholes and systems
Mereology
Also known as: Part–whole theory, Theory of parts and wholes
German: Mereologie
In philosophy and formal ontology, mereology is the theory of the relations between parts and wholes — what it means to be part of something, how parts overlap, and when several parts make up a whole.
- Thinking models
- Philosophy
- Logic
In one sentence
Mereology is the theory of parts and wholes: being part of, overlapping, and composing a whole.
Example
Is the safety relay part of the control cabinet, of the safety function, or of both? A parts list answers the first question, a safety function description the second. Mixing the two part–whole relations confuses readers of both.
How it applies
- Basic relations: Parthood (x is part of y), proper parthood (part but not identical), overlap (x and y share a part) and composition or fusion (several things together make up a whole). Formal mereology states axioms for them, for example that parthood is transitive.
- Origins: Husserl’s third Logical Investigation (1901) analyzed dependent and independent parts; Leśniewski developed the first formal system and coined the name, from Greek meros, part.
- Not one relation: Everyday “part of” covers different relations — component of an assembly, member of a group, portion of a substance, phase of a process. Transitivity can fail when they are mixed (“the handle is part of the door, the door is part of the house, the handle is part of the house” works; many other chains don’t).
- Technical documentation: Product structures, topic hierarchies and DITA maps are part–whole structures of different kinds. Saying which part–whole relation a hierarchy expresses — assembly, function, document — keeps components reusable instead of tied to one structure.
Mereology vs. taxonomy
A taxonomy orders kinds (a light curtain is a protective device); mereology orders parts (a light curtain is part of the safeguarding of zone Z2). Information models need both relations and should never use one for the other.
How to read this pool
Entries, context cards and transitional topics
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