The rest of this paper is structured as follows. Section 2 considers brieﬂy what has been said against the idea of second person thought. It notes a couple of lines of argument which might ground the denial that there is any such thing and suggests that they are far from conclusive. Section 3 considers language learning. A widely accepted picture of how it proceeds, together with a few empirical observations, provides initial support for the idea that there is something which deserves to be called ‘second person thought’. Section 4 turns to co-operation and identiﬁes a distinctive pattern of practical reasoning, ﬁrst person plural reasoning, plausibly to be found in it. Section 5 then sketches in broadbrush terms three increasingly elaborated forms of co-operative activity, which we shall call side-by-side co-operation, face-to-face co-operation and ‘you’-using co-operation. Finally, in the light of the ideas of Sections 4 and 5, Section 6 offers a proposal on one way to understand ‘second person thought’ and to identify at least some instances of it. It remarks on some features of the concepts in terms of which the proposal is framed (common knowledge and co-operation) and notes some of the many further issues which arise if this framework of ideas is accepted.