 Suppose the larger system has wave function \(\Psi = \Psi(q) = \Psi(x, y)\). According to Bohmian mechanics, the larger system is then completely described by \(\Psi\), evolving according to Schrödinger’s equation, together with \(X\) and \(Y\). The question then arises—and it is a critical question—as to what should be meant by the wave function of the subsystem.