BUT informal fallacies are not limited to problematic arguments but also to problematic techniques in many contexts of rhetoric and argumentation. Thus, it need not be logically invalid to be fallacious in usage. (Source)
If I rewrite the last sentence wholly positively, does it imply statements (not necessairly arguments) that are logically valid but fallacious? Really? Please explain? What's this paradox called? Examples please?
We do not usually call a statement »valid« or »fallacious«. A statement is true or false. An argument on the other hand is valid, if it complies with the rules of logic. But this is not enough: »All humans are stupid. Fred is a human. Therefore Fred is stupid.« is a valid argument. But we might doubt the premise that all humans are stupid. If an argument is valid and its premises are true, we call the argument sound.
Note that a sound argument does not require the premises to be proven. This is in fact often impossible. Instead they should be plausible. But deciding on the plausibility of a premise is often subjective. That's where (informal) fallacies come in. They are rhetorical devices employed to make a premise sound plausible. Therefore they are problematic. But the argument itself can still be valid or even perfectly sound. Even the conclusion can be true. Fallacies are just a warning sign that you should double check the premises.
