 There is a rather obvious answer for this, a natural function of \(x\) that suitably incorporates the objective structure at hand, namely the conditional wave function   \[\psi(x) = \Psi(x, Y)\]   obtained by plugging the actual configuration of the environment into the wave function of the larger system. (This definition is appropriate only for scalar wave functions; for particles with spin the situation would be a little more complicated.) It then follows immediately that the configuration of the subsystem obeys  the guiding equation  with the conditional wave function on its right-hand side.