Thinking and using a language are two different but similar activities. Thinking about thinking and thinking about language use have been two major strands in the history of philosophy. One of the principal similarities is that they are both rational activities. As a result, the ability to think and the ability to use a language require being able to recognise and respond to reasons. However, there is a further feature of these activities: we humans are able to have explicit knowledge of how those activities work and what is done by performances in those activities. Thus, theorists face at least two constraints: 1. An account of a rational activity must be compatible with the possibility of agents engaging in that activity. 2. Having described an activity, it must be possible to have knowledge of an activity which is correctly described like that. There are a variety of accounts of how thinking works and how using a language works, and further variation in accounts of what is involved in explicit understanding of particular performances. These accounts can be distinguished by their views of the nature of the reasons that govern performances in that activity and by their views of the way a description of the activity relates to the way the activity proceeds. I argue that any description of thinking or language use requires showing how the truth conditions of thoughts/sentences are determined, and how the truth values of thoughts/sentences affects the way the activity proceeds. I then argue that in order to have explicit knowledge of what we do, truth has to be a substantial evaluative property of uses of language, and furthermore a truth conditional theory of meaning has to be taken as the description of the rationality of using a language. The big result is that, because in understanding language we understand truth, the philosophy of language is first philosophy.