4 reformulated the first question

# Can concreteinfinitefirst-order categories be definedspecified other than as categories of modelsofsometheory?

I am glad to see that a general question like Is there a relationship between model theory and category theory? receives quite a lot attention and no down-votes for being too general and unspecific. So I feel encouraged to pose some follow-up questions (firstly restricted to first-order model theory).

### Preliminaries

I hope the following statements are sufficiently sensible, precise and correct.

• Each first-order theory $T$ with signature $\sigma$ unambigously defines a class of (ZF-)models.

• This class of models of $T$ together with the $\sigma$-homomorphisms form a category (the category of models of $T$).

• Two first-order theories with two arbitrary signatures may define equivalent categories of models.

Definition: A first-order theory $T$ provides a model of a category $C$ if the category of models of $T$ is equivalent to $C$.

• Each category $C$ defines a (possibly empty) set of first-order theories: the set of all $T$ which provide a model of $C$.

### Questions

[Remark: I had to work this question over, since it seemed to be ill-posed.]

Old version: Can infinite concrete categories be specified other than as categories of (ZF-)models of some (possibly higher-order) theory? Examples?

New version (explicitly restricted to first-order theories):

Given an infinite category of models of a first-order theory $T$. Can this category - or one equivalent to it - be specified/represented/given independently of any first-order theory $T$ and its (ZF-)models?

Remark: $T$ of course can be specified/represented/given independently of its models: as a set of formulas.

Why is the notion of models of a (concrete) category so uncommon? (Maybe because the answer to the first question is "No"?)

Is there a genuine model-theoretic notion of two equivalent theories if these have two arbitrary signatures?

3 added 9 characters in body

I am glad to see that a general question like Is there a relationship between model theory and category theory? receives quite a lot attention and no down-votes for being too general and unspecific. So I feel encouraged to pose some follow-up questions (firstly restricted to first-order model theory).

### Preliminaries

I hope the following statements are sufficiently sensible, precise and correct.

• Each first-order theory $T$ with signature $\sigma$ unambigously defines a class of (ZF-)models.

• This class of models of $T$ together with the $\sigma$-homomorphisms form a category (the category of models of $T$).

• Two first-order theories with two arbitrary signatures may define equivalent categories of models.

Definition: A first-order theory $T$ provides a model of a category $C$ if the category of models of $T$ is equivalent to $C$.

• Each category $C$ defines a (possibly empty) set of first-order theories: the set of all $T$ which provide a model of $C$.

### Questions

Can infinite concrete categories be specified other than as categories of (ZF-)models of some (possibly higher-order) theory? Examples?

Why is the notion of models of a (concrete) category so uncommon? (Maybe because the answer to the first question is "No"?)

Is there a genuine model-theoretic notion of two equivalent theories if these have two arbitrary signatures?

2 added 69 characters in body; edited tags

I am glad to see that a general question like Is there a relationship between model theory and category theory? receives quite a lot attention and no down-votes for being too general and unspecific. So I feel encouraged to pose some follow-up questions (firstly restricted to first-order model theory).

### Preliminaries

I hope the following statements are sufficiently sensible, precise and correct.

• Each first-order theory $T$ with signature $\sigma$ unambigously defines a class of (ZF-)models.

• This class of models of $T$ together with the $\sigma$-homomorphisms form a category (the category of models of $T$).

• Two first-order theories with two arbitrary signatures may define equivalent categories of models.

Definition: A first-order theory $T$ provides a model of a category $C$ if the category of models of $T$ is equivalent to $C$.

• Each category $C$ defines a (possibly empty) set of first-order theories: the set of all $T$ which provide a model of $C$.

### Questions

Can concrete categories be specified other than as categories of (ZF-)models of some (possibly higher-order) theory? Examples?

Why is the notion of models of a (concrete) category so uncommon? (Maybe because the answer to the first question is "No"?)

Is there a genuine model-theoretic notion of two equivalent theories if these have two arbitrary signatures?

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