Language learners use the data in their environment in order to infer the grammatical system that produced that data. Yang (2018) makes the important point that this process requires integrating learners’ experiences with their current linguistic knowledge. A complete theory of language acquisition must explain how learners leverage their developing knowledge in order to draw further inferences on the basis of new data. As Yang and others have argued, the fact that input plays a role in learning is orthogonal to the question of whether language acquisition is primarily knowledge-driven or data-driven (J. A. Fodor, 1966; Lidz & Gagliardi, 2015; Lightfoot, 1991; Wexler & Culicover, 1980). Learning from data is not incompatible with approaches that attribute rich initial linguistic knowledge to the learner. On the contrary, such approaches must still account for how knowledge guides learners in using their data to infer a grammar.