Argument Validator - Analyze Logic, Detect Fallacies & Check Validity
Validate arguments by identifying premises, conclusions, logical fallacies, and structural validity. Free online argument analysis tool.
Write one premise per line. Use logical connectors like because, therefore, ifβ¦then, since, hence.
Enter an argument above and click Validate Argument to begin analysis.
About
Logical arguments fail silently. A structurally invalid argument can sound persuasive. An argument riddled with fallacies can pass unnoticed in debate, academic writing, or legal reasoning. This tool parses natural-language arguments into their component parts - P1, P2, β¦, Pn (premises) and C (conclusion) - then evaluates structural validity against standard inference rules such as modus ponens and disjunctive syllogism. It cross-references over 25 catalogued formal and informal fallacies. The strength score (0 - 100) reflects structural integrity, fallacy presence, and premise verifiability.
Limitations: The tool approximates natural-language parsing. It cannot verify empirical truth of premises. Ambiguous phrasing or implicit premises may reduce detection accuracy. For best results, state each premise on its own line and use explicit logical connectors (because, therefore, ifβ¦then). This is a structural analysis tool, not a truth oracle.
Formulas
The argument strength score S is a weighted composite of three dimensions:
Where Sstruct = structural quality score (0 - 100). Awards points for: identifiable conclusion (30 pts), at least 2 premises (30 pts), logical connector usage (20 pts), and valid inference pattern match (20 pts). Sfallacy = fallacy-free score. Starts at 100 and subtracts 20 per detected fallacy, floored at 0. Spremise = premise support score. Evaluates whether premises contain verifiable claims vs. subjective assertions. Each verifiable premise adds 100n where n = total premise count.
Validity check applies pattern matching against standard inference rules:
Reference Data
| Fallacy | Type | Pattern | Example |
|---|---|---|---|
| Ad Hominem | Informal | Attacks the person, not the argument | "You're wrong because you're biased." |
| Straw Man | Informal | Misrepresents the opposing position | "So you're saying we should have no rules at all?" |
| Appeal to Authority | Informal | Cites authority outside relevant expertise | "A celebrity says this diet works." |
| False Dilemma | Informal | Presents only two options when more exist | "Either you support X or you support chaos." |
| Slippery Slope | Informal | Assumes chain of events without justification | "If we allow A, then B, C, and D will follow." |
| Circular Reasoning | Informal | Conclusion restates a premise | "It's true because it's a fact." |
| Red Herring | Informal | Introduces irrelevant topic to divert | "Why worry about X when Y is happening?" |
| Appeal to Emotion | Informal | Uses emotional manipulation over evidence | "Think of the children!" |
| Bandwagon | Informal | Appeals to popularity as proof | "Everyone believes this, so it must be true." |
| Hasty Generalization | Informal | Concludes from insufficient sample | "I met two rude locals, so everyone there is rude." |
| Tu Quoque | Informal | Deflects by accusing hypocrisy | "You do the same thing!" |
| Appeal to Nature | Informal | Assumes natural equals good | "It's natural, therefore it's safe." |
| Appeal to Ignorance | Informal | Claims truth from lack of disproof | "No one has proven it wrong, so it's true." |
| Equivocation | Informal | Shifts word meaning mid-argument | "The sign said "fine for parking here." So it's fine to park." |
| Loaded Question | Informal | Presupposes unproven assumption | "When did you stop cheating?" |
| No True Scotsman | Informal | Redefines group to exclude counterexamples | "No real expert would disagree." |
| Post Hoc | Informal | Assumes causation from sequence | "After the policy, crime dropped - so the policy worked." |
| Affirming the Consequent | Formal | P β Q, Q β΄ P | "If it rains the ground is wet. The ground is wet, so it rained." |
| Denying the Antecedent | Formal | P β Q, Β¬P β΄ Β¬Q | "If it rains the ground is wet. It didn't rain, so the ground isn't wet." |
| Undistributed Middle | Formal | Middle term not distributed in syllogism | "All dogs are animals. All cats are animals. So all dogs are cats." |
| Illicit Major | Formal | Major term distributed in conclusion but not premise | "All roses are flowers. No daisies are roses. So no daisies are flowers." |
| False Cause | Informal | Incorrectly identifies causal relationship | "Ice cream sales and drowning correlate, so ice cream causes drowning." |
| Composition | Informal | Assumes whole has properties of parts | "Each brick is light, so the wall is light." |
| Division | Informal | Assumes parts have properties of whole | "The team is great, so every player is great." |
| Genetic Fallacy | Informal | Judges by origin instead of content | "That idea came from a flawed study, so it's wrong." |