 In one sense, it seems quite straightforward to give a deductive justification for some favored set of rules of inference. It can be shown, for example, that if the premises of an application of Modus Ponens are true then the conclusion must also be true. The problem, at least potentially, is that such justifications typically make use of the very rule which they seek to justify. In the above case: if MP is applied to true premises then the conclusion is true; MP is applied to true premises; hence the conclusion is true. Haack (1976) and others have debated whether the circularity here is vicious or not. One crucial consideration is whether analogous ‘justifications’ can be given for invalid rules, for example Prior’s introduction and elimination rules for ‘tonk,’ that also have this feature of using a rule to justify  itself.[5]  (A closely related issue can be traced back to Lewis Carroll and his classic (1895) paper.)