Human thought can be characterised as being situated in the 'space of reasons'. That is to say that human thought is guided by the norms of theoretical and practical rationality which, in turn, enable autonomous thinking. But how do children learn to navigate the space of reasons? Building on the work of Tugendhat and Bakhurst, among others, we argue, first, that this process involves the acquisition of propositional thought and that it is an essentially social one in which both children and adults take an active role. Second, and more specifically, by drawing on Brandom's theory of meaning, we provide a philosophical analysis of empirical findings with respect to the efficacy of 'Sustained Shared Thinking' (SST) in early educational settings. We argue that the efficacy of SST is based on its emphasising, modelling and practising those elements of communication that are essential for gaining proficiency in 'playing the game of giving and asking for reasons', namely drawing inferences between propositions and concepts and providing justifications for one's assertions. This is achieved by making explicit the inferential relationships between concepts and propositions and by the use of modal and doxastic markers that function as an invitation to a joint search for reasons.