# Definition:Morphisms-Only Metacategory

## Definition

A **morphisms-only metacategory** is a metamodel for the language of category theory subject to the following axioms:

\((\text {MOCT} 0)\) | $:$ | \(\ds \forall x, y, z, z':\) | \(\ds \paren {\map {R_\circ} {x, y, z} \land \map {R_\circ} {x, y, z'} } \implies z = z' \) | $\circ$ is a partial mapping in two variables | |||||

\((\text {MOCT} 1)\) | $:$ | \(\ds \forall x, y:\) | \(\ds \Dom x = \Cdm y \iff \exists z: \map {R_\circ} {x, y, z} \) | domain of composition $\circ$ | |||||

\((\text {MOCT} 2)\) | $:$ | \(\ds \forall x, y, z:\) | \(\ds \map {R_\circ} {x, y, z} \implies \paren {\Dom z = \Dom y \land \Cdm z = \Cdm x} \) | Domain and codomain of a composite $z = x \circ y$ | |||||

\((\text {MOCT} 3)\) | $:$ | \(\ds \forall x, y, z, a, b:\) | \(\ds \map {R_\circ} {x, y, a} \land \map {R_\circ} {y, z, b} \implies \paren {\exists w: \map {R_\circ} {x, b, w} \land \map {R_\circ} {a, z, w} } \) | $\circ$ is associative | |||||

\((\text {MOCT} 4)\) | $:$ | \(\ds \forall x:\) | \(\ds \map {R_\circ} {x, \Dom x, x} \land \map {R_\circ} {\Cdm x, x, x} \) | Left identity and right identity for $\circ$ |

## Explanation

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A **morphisms-only metacategory** can thus be described as follows.

Let $\mathbf C_1$ be a collection of objects called **morphisms**.

Let $\mathbf C_2$ be the collection of pairs of morphisms $\tuple {g, f}$ with $\operatorname {cod} f = \operatorname {dom} g$; write $\map {\mathbf C_2} {g, f}$ to express that $\tuple {g, f}$ is a member of $\mathbf C_2$.

By $(MOCT1)$, we see that $\map {\mathbf C_2} {g, f}$ thus is an abbreviation of the statement $\exists h: \map {R_\circ} {g, f, h}$.

Let $\circ$ be an operation symbol which must assign to every pair of morphisms $\tuple {g, f}$ in $\mathbf C_2$ a morphism $g \circ f$, called the **composition of $g$ with $f$**.

We see that $g \circ f$ satisfies $\map {R_\circ} {g, f, g \circ f}$; by axiom $(MOCT0)$, it is unique.

Axioms $(MOCT1)$ up to $(MOCT3)$ combine to ensure that $h \circ \paren {g \circ f}$ is defined if and only if $\paren {h \circ g} \circ f$ is, and that they are equal when this is the case.

Finally, axiom $(MOCT4)$ entails the existence and uniqueness of left- and right-identities for $\circ$.

## Also see

## Sources

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