 That paper is about conversational dynamics. Lewis develops an extended analogy between the role of context in a conversation and the role of score in a baseball game. One central role of the score is to keep a record of what has already happened. In that way, score is influenced by what happens on the field, or in the conversation. But the causal influence runs in the other way as well. Some events on the field are influenced by the score. You’re only out after the third strike, for example. Similarly, Lewis holds that context (or the conversational score) can influence, or even be partially constitutive of, what happens in the conversation. If I say “None of the cats are afraid of Barney”, which cats I’ve managed to talk about depends on which cats are conversationally salient. And in saying this, I’ve made Barney salient, so the score changes in that respect. That change matters; now I can denote Barney by “he”.