New posts in First Order Logic

Proof that there exist no finite axiomatic system with Compactness Theorem

**Ended Competition:** What is the shortest proof of $\exists x \forall y (D(x) \to D(y)) $?

What is the purpose of free variables in first order logic?

Relationship between propositional logic, first-order logic, second-order logic higher-order logic, and type theory

categorical interpretation of quantification

Is the compactness theorem (from mathematical logic) equivalent to the Axiom of Choice?

Predicate vs function

What is the so-called eigenvariable or parameter in natural deduction?