The purpose of this article is to show that Piaget's use of the equilibrium principle cannot explain the possibility of correct understanding. That is, it cannot explain the possibility of knowledge, as opposed to simple change in belief. To make the argument, I begin by describing Piaget's explanatory model, which is known as the equilibrium principle. I then argue that correct understanding, or knowledge of any x as a case of y, requires a concept of correctness, i.e., the recognition that (...) words and concepts apply under some conditions but not others. I try to show that because he uses the equilibrium principle as a basis for his explanation, Piaget cannot explain how a concept of correctness is acquired. Finally, I argue that to explain the possibility of knowledge, one must show how the conditions for word and concept application are determined by a community of language users. Again, I claim that Piaget's use of the equilibrium model precludes such an account. (shrink)