Semantics seeks to explain knowledge of meaning, as well as how linguistic expressions are about something. But there is a tension between these two goals. The issue is to what extent meanings depend on aspects of the world external to the minds of speakers and how such independence is compatible with full knowledge of meanings. Philosophical work on this topic is preoccupied with thought experiments due to Hilary Putnam and Tyler Burge. Externalists seek to reconcile the tension by individuating mental states in terms that refer to the external world. Internalists distinguish two notions of meaning: What you grasp when you understand a sentence is one thing, and what determines the truth-conditions of that sentence is another.