Rigorous Reasoning
← Reasoning Lab

Formal Logic · Free

Proof Constructor

Build natural-deduction proofs step by step. Each inference rule is checked semantically — the validator parses your formulas and verifies the cited lines actually license the move you named, not just that you typed something in the slot.

Used in: Propositional Logic · Predicate Logic · Modal Logic · Natural Deduction lessons. Load a worked example from the dropdown to see what a complete proof looks like, or pick a practice template and try it.

Where to go next

Argument EditorWrite the argument in plain English first — the editor symbolizes each line, checks validity, and verifies cited rules (MP, HS, UI…) as you type.Open tool →Truth Table BuilderVerify a formula's truth table before (or alongside) building a proof — useful for sanity-checking what you're trying to derive.Open tool →