In the formulation of the de Broglie–Bohm theory, there is only a wavefunction for the entire universe (which always evolves by the Schrödinger equation). It should, however, be noted that the "universe" is simply the system limited by the same boundary conditions used to solve the Schrödinger equation. However, once the theory is formulated, it is convenient to introduce a notion of wavefunction also for subsystems of the universe. Let us write the wavefunction of the universe as formula_45, where formula_46 denotes the configuration variables associated to some subsystem (I) of the universe, and formula_47 denotes the remaining configuration variables. Denote respectively by formula_48 and formula_49 the actual configuration of subsystem (I) and of the rest of the universe. For simplicity, we consider here only the spinless case. The "conditional wavefunction" of subsystem (I) is defined by