 Gerald Massey (1981) has voiced a strong objection to fallacy theory and the teaching of fallacies. He argues that there is no theory of invalidity—no systematic way to show that an argument is invalid other than to show that it has true premises and a false conclusion (1981, 164). Hence, there is an asymmetry between proving arguments valid and proving them invalid: they are valid if they can be shown to be an instance of a valid form, but they are not proved invalid by showing that they are an instance of an invalid form, because both valid and invalid arguments instantiate invalid forms. Thus, showing that a natural language argument is an instance of an invalid form does not preclude the possibility that it is also an instance of a valid form, and therefore valid. Since upholders of SDF maintain that fallacies are invalid arguments, Massey’s asymmetry thesis has the consequence that no argument can be convicted of being a fallacy on logical  grounds.[6]