Principle
Valid vs. true
Also known as validity, soundness, valid argument, sound argument or cogent argument
True and valid describe different things. A statement is true or false: it matches the world or it doesn’t. An argument is valid or invalid: it’s valid when its premises, if they were true, would guarantee its conclusion. Validity is about the connection, not the facts.
That’s why every error on this site comes with the same warning: a flawed argument can still reach a true conclusion. Finding the error tells you the argument doesn’t support its conclusion. It doesn’t tell you the conclusion is false.
Example
“All birds can fly. Penguins are birds. So penguins can fly.”
This argument is valid: if both premises were true, the conclusion would have to be. It’s still wrong, because the first premise is false. Now compare:
“Some birds can’t fly. Penguins are birds. So penguins can’t fly.”
Here the conclusion is true, but the argument is invalid. “Some birds can’t fly” doesn’t show that penguins are among them, so the argument got the right answer by luck.
The four combinations
| Conclusion true | Conclusion false | |
|---|---|---|
| Valid argument | Possible (and if the premises are true too, the argument is sound) | Possible, but only if a premise is false |
| Invalid argument | Possible: right answer, bad reasoning | Possible |
Only one combination is impossible: a valid argument with all true premises and a false conclusion. That impossibility is exactly what “valid” means.
Sound: valid *and* true
An argument that is valid and has all true premises is called sound. A sound argument’s conclusion must be true. So checking an argument always means asking two separate questions:
- Does the conclusion follow? (Is it valid?) This is where formal fallacies fail.
- Are the premises true? This is a question about the world, not about logic.
Beyond strict logic: strong and cogent
Most everyday reasoning doesn’t aim for a guarantee. “The last forty trains were on time, so this one probably will be” can’t be valid, because the next train could always be late, but it can be good. For arguments like this, logic textbooks use different words:
- Strong: if the premises are true, the conclusion is probably true.
- Cogent: strong, and the premises actually are true.
Informal fallacies and cognitive biases usually fail here: the reasons given are irrelevant, too weak, or weighed wrongly. Even so, the same warning applies: weak reasoning can still land on the truth.
Naming the error isn’t refuting the conclusion
Concluding that a claim is false because someone argued for it badly is itself a mistake, sometimes called the argument from fallacy or the “fallacy fallacy”.
“You only think the café closes at 9 because everyone says so. That’s an appeal to popularity, so it must close later.”
The argument may be bad, and the café may still close at 9. The right response to a flawed argument is “that doesn’t show it”, not “so it’s false”. To show it’s false, you need evidence against the claim itself.
Sources
- Irving M. Copi, Carl Cohen and Kenneth McMahon (2011). Introduction to Logic (14th edition). Pearson.
- Patrick J. Hurley and Lori Watson (2018). A Concise Introduction to Logic (13th edition). Cengage Learning.
- Robert Arp, Steven Barbone and Michael Bruce (eds.) (2018). Bad Arguments: 100 of the Most Important Fallacies in Western Philosophy. Wiley-Blackwell.
Last reviewed 2026-09-13.