Wittgenstein’s “private language argument” we think of as something in the Philosophical Investigations, although we may disagree about where exactly in that work the argument is found, and also about what sort of argument it is. We can find earlier adumbrations of the argument, or something like it, in Wittgenstein’s writings of the 1930s, and in his notes for lectures during those years. We see the topic of privacy, of our capacity to speak or think about our own private sensations, as a topic of Wittgenstein’s later philosophy. And we take the appearance of the topic as indicative of a shift in Wittgenstein’s philosophical interests to topics within the philosophy of mind, not of interest to him in the Tractatus; further, we may take his treatment of the topic as an illustration of the fundamental shifts in his overall philosophical position. Michael Dummett, for example, takes Wittgenstein’s private language argument to be deeply anti-realist, and thus at a great distance from Wittgenstein’s realism in the Tractatus. I shall argue that there is a private language argument in the Tractatus, closely related to the argument in the Investigations, although also different from it in important respects.