In the Philosophical Investigations, §258, Wittgenstein invites us to consider a diarist recording the occurrences of a particular kind of sensation, which he proposes to call by some name. Wittgenstein argues that in so far as the sensation is thought of as purely private, disconnected from behaviour and symptoms of its occurrence, the procedure is a pantomime, since the subject cannot give himself a determinate understanding of what he is doing. There is no distinction between being right, in recording an ostensible recurrence, and seeming to be right. The argument is part of an attack on the Cartesian way of thinking of the mind as an inner theatre, whose show is known in a privileged and unique way by its possessor.

One way of thinking of Wittgenstein’s strategy is by seeing it as suggesting that just as other persons and their minds become a problem for the Cartesian, so should our own past minds. ‘Always get rid of the idea of a private object in this way: assume that it constantly changes, but that you do not notice the change because your memory constantly deceives you’ (Philosophical Investigations, Pt. II, xi). The vacuous nature of this hypothesis shows that it is illusory to suppose that there is something to be right or wrong about. Although the argument has been immensely influential and widely accepted, commentators have disagreed on whether it has a verificationist premise, and if so whether this decreases its force. There has also existed uncertainty over whether it leads to a behaviourist view of the mind. See also functionalism, inverted spectrum, qualia.