A fallacy is the use of poor, or invalid reasoning for the construction of an argument. It is also used to refer to "an argument which appears to be correct but is not." If an argument is fallacious it does not necessarily mean the conclusion is false.Fallacies are commonly divided into those that are formal and those that are informal. A formal fallacy can neatly be expressed in standard system of logic, for example propositional logic. Conversely, an informal fallacy originates in an other error in reasoning than an improper logical form. Arguments committing informal fallacies may be formally valid, but still be fallacious.Fallacies of presumption fail to prove the conclusion by assuming the conclusion in the proof. Fallacies of weak inference fail to prove the conclusion due to insufficient evidence. Fallacies of distraction fail to prove the conclusion due to irrelevant evidence, like emotion. Fallacies of ambiguity fail to prove the conclusion due to vagueness in words, phrases, or grammar.Some fallacies are committed intentionally (to manipulate or persuade by deception), others unintentionally due to carelessness or ignorance.