Hierarchically, the knowledge level lies above the symbol level where all of the knowledge in the system is represented. In other words, a tacit assumption in AI is that a knowledge-level system must contain a symbol system (i.e. this is simply a re-statement of the physical symbol system hypothesis). According to Newell's unified theory of cognition, the symbol level corresponds to the cognitive band and the knowledge level to the rational band. However, in the latter case, humans only approximate a knowledge-level system, due to the constraints imposed by bounded rationality. In Unified Theories of Cognition, Newell suggested that psychology is the study of this approximation, when the architecture "shows through" and behavior is not mediated by knowledge alone.