I shall show that the main argument forms of Wittgenstein's Tractatus and Kant's Critique of Pure Reason are Modus Tollens. I shall then argue that the main arguments of both books beg the question by addressing only one sub-argument in each, although it is still in controversy whether begging the question is a genuine fallacy.

This paper discusses Lippert-Rasmussen’s [Philosophical Studies 104, (2001) 123–141] claim that there are reasonable question-begging arguments. It is first argued that his arguments devalue the distinction between justifiable and justified beliefs, a distinction that is important for the fallacy theory. Second, it is argued that the role of the argument in the discussed cases can be questioned. In addition, the role of second order beliefs is discussed.