In my question yesterday I asked about the definition and usage of the word "model", for which I was told the following definition:
A formula of propositional logic is true under an interpretation iff the interpretation assigns the truth value T to that formula. If a formula is true under an interpretation, then that interpretation is called a model of that formula.
An interpetation satisfies a formula φ iff it is a model of the formula. Same for a set $\Gamma$ of formulas, if an interpretation satisfies all the formulas in $\Gamma$ then it is a model of $\Gamma$. Then if we say $\Gamma \vDash \varphi$ then $\varphi$ is a semantic consequence of $\Gamma$ / is satisfied in all "models" of $\Gamma$ (true whenever everything in $\Gamma$ is true).
All good so far. But when I get really confused when I see posts like these where they use "models" as a word meaning "satisfies all the axioms in a theory."
From Asaf's answer:
Models are structures, and structures are models. But when we say "model" we mean that there is a particular theory which holds in the structure, and when we say "structure" we are mainly interested in an arbitrary interpretation of the language.
From Metin's answer:
A structure is a set with some interpretable symbols(constants, relations and functions) within a fixed language. You do not ask for more from a structure.
A model (of a theory) is a structure which satisfies the axioms of the theory. It makes more "structural sense"...
And then from one of Asaf's comments:
When I say "a model of 𝖹𝖥𝖢" they also know, immediately, that I am talking about a model which satisfies all the axioms of 𝖹𝖥𝖢.
So I am unclear on which definition is correct or if this is the same word being used in two different ways or if this is "logical model" versus "semantic model" or something else entirely.
On the one hand we seem to have "model" meaning "any interpretation that satisfies a formula or set of formulas." On the other hand we have "model" meaning "Interpretation that satisfies all the axioms of a theory".
From the Wiki article on Structure (mathematical logic) we see also:
For a given theory in model theory, a structure is called a model, if it satisfies the defining axioms of that theory.
So which is it exactly? Does "model" have a different meaning in propositional logic compared to model theory or first-order logic or set theory etc?