Meaning, rules and conversation

Wittgenstein writes: ‘To understand a sentence means to understand a language’. My question is: what is a language, and what is its importance to the idea of understanding what someone has said? Familiar ways of developing Wittgenstein’s ‘rule-following considerations’, along with the idea that the notion of a ‘correctness condition’ must be central to any account of meaning, do not throw light on the idea of a language. If we give central place to the idea that understanding a sentence involves grasping its logical relations with other sentences we must remember that it is the things that people say that stand in logical relations with each other, and that this is just one instance of the more general point that in a conversation what one person says may bear on what another says. The notion of a ‘conversation’ may vary in its temporal stretch. Those with whom I share a language are those with whose words what I say may be connected: connected in a way analogous to that in which the remarks in a conversation are connected.