New posts in Natural Deduction

Proving De Morgan's Law with Natural Deduction

How do I prove the tautology $\vdash((p\rightarrow q)\rightarrow p)\rightarrow p$ using natural deduction?

Why isn't Modus Ponens valid here

How to prove that $P \rightarrow Q$ is equivalent with $\neg P \lor Q $?

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

Natural deduction proof of $(\alpha\to\beta)\to(\beta\to\gamma)\to(\alpha\to\gamma)$

Prove that $\vdash p \lor \lnot p$ is true using natural deduction

What's wrong with this inference in natural deduction?

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