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Models for predicate logic

Before we dive into proving statements in predicate logic, we equip ourselves with a useful tool to decide if a formula is not provable.

For propositional logic, we used truth assignments and truth tables as a quick tool to establish the lack of a proof. This was only possible because we established soundness of truth assignments: if we could establish $X \vdash Y$, then we know that $X \models Y$, ie every truth assignment that makes $X$ true will also make $Y$ true.

Propositions are simpler than predicates though. For predicate logic, the interpretations have more moving parts. Our domain of discourse, or source for inputs of the variables, needs to be assigned a concrete set of values. For example, we could consider $\mathbb{N}$ as our concrete set. Then, each function $f(x)$ should get assigned to a function whose inputs are from $\mathbb{N}$ and whose outputs lie in $\mathbb{N}$, like $f(x) = x+1$.

Each predicate $A(x)$ may be true for some numbers $x$ and false for others. Thus, we need to assign a set of values for which $A(x)$ is true and assign false for the others. For example, we could assign $A(x)$ to the set of even natural numbers. Meaning, $A(x)$ is true for $x$ even and false $x$ odd.

Remember that predicate logic is purely about symbolic manipulation respecting a set of rules. Through models, we imbue some meaning and familiarity to the symbols.

Our rules for propogating truth and falsity through our familiar connectives $\to, \land, \lor, \neg, \leftrightarrow$. We need ways to assign values to $\forall$ and $\exists$.

These are built into the motivations for these symbols. We declare that $\forall x~ A(x)$ evaluates to true if $A(x)$ is true for all values of $x$ in our model. It is false otherwise. For example, we if use $\mathbb{N}$ and interpret $A(x)$ to be $x > 5$. Then, $\forall x~ A(x)$ would evaluate to false. But if we interpret $A(x)$ to be $x^2-x \geq 0$ then we would get true.

Similarly, $\exists x~ A(x)$ evaluates to true if there at least one value in our model making $A(x)$ true and is otherwise false.


With our more complex models, we can extend $X \models Y$ to predicate logic.

Definition. We say that a formula $Y$ is a logical consequence of a formula $X$ if, in any model, whenever $X$ is true, then $Y$ is true.

So to show that $X \not \models Y$ we just need to locate one model where $X$ evaluates to true while $Y$ to value for some value.

Predicate logic is sound similar to propositional logic.

Theorem. If $X \vdash Y$, then $X \models Y$.

Like with propositional logic, this gives a powerful means to check that $X \not \vdash Y$: just find one model demonstrating $X \not \models Y$.


Amazingly, the converse of the previous theorem continues to hold.

Theorem. If $X \models Y$, then $X \vdash Y$.

Completeness of predicate logic is more involved than that for propositional logic. It was first proven by Kurt Gödel in his 1929 thesis. We won’t go into any details and will not use completeness in any way moving forward. But, hopefully, you are well equiped to appreciate the statement.