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Proofs
This proof editor and checker demonstrates Fitch-style natural deduction systems used in introductory logic textbooks.
About Proofs
Proofs is a web-based editor and checker for Fitch-style natural deduction proofs. It uses the system from forall x: Calgary and also supports rules used in the forall x: Cambridge remix.
Users can create TFL or FOL problems by entering premises and a conclusion, or open sample truth-functional and first-order exercise sets. Proof lines accept several typed symbols for connectives, quantifiers and contradiction, and non-premise lines include a field for rule justifications with line references.
Key features
- Create custom TFL or FOL problems with premises and conclusion
- Check Fitch-style natural deduction proofs line by line
- Includes sample truth-functional and first-order exercise sets
- Accepts multiple typed symbols for each logical operator
- Add and nest subproofs with on-screen line controls
- Enter rule justifications with cited line numbers per line
Best for: Students practicing Fitch-style natural deduction from forall x: Calgary or the Cambridge remix.
Good for
- Supports both Calgary rules and Cambridge remix rules
- Includes sample TFL and FOL exercise sets
- Accepts many alternative symbols for connectives, quantifiers and contradiction
Watch out for
- Follows the forall x: Calgary system, which differs from the original forall x version
- Requires specific TFL and FOL sentence formats for atomic sentences
Proofs at a glance
- Website
- proofs.openlogicproject.org
- Category
- Education & Learning › History & Humanities
- Type
- Website
- Pricing
- Not confirmed. Check the site
- Platforms
- Web
- Open source
- Not stated
- Language
- English
- Last checked
- Oct 5, 2026 · Online when checked
- Listed
- Oct 5, 2026
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Proofs FAQ
Which proof system does Proofs check?
The forall x: Calgary system, with support for rules used in the forall x: Cambridge remix.
How do I start a custom problem?
Select TFL or FOL, enter premises separated by commas or semicolons, enter a conclusion, then create the problem.
How do I justify a proof line?
Click the cell to the right of the line and enter the rule and line numbers, such as "&I 1,2".
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