A fallacy is an argument that uses poor reasoning. An argument can be fallacious whether or not its conclusion is true. A fallacy can be either formal or informal. An error that stems from a poor logical form is sometimes called a formal fallacy or simply an invalid argument. An informal fallacy is an error in reasoning that does not originate in improper logical form. Arguments committing informal fallacies may be formally valid, but still 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.