 In the previous section we indicated that collapse of the wave function can be regarded in Bohmian mechanics as a pragmatic affair, that the collapse of the wave function is an effective collapse.  However, there is a sense in which the collapse of the wave function in Bohmian mechanics is more than a matter of convenience, a sense in which the collapse is actual and not merely effective. Indeed, if we focus on the appropriate notion of the wave function, not of the composite of system and apparatus—which strictly speaking remains a superposition if the composite is treated as closed during the measurement process—but of the system itself, we find that for Bohmian mechanics this wave function does in fact collapse, precisely as the quantum formalism says. The key element here is the notion of the conditional wave function of a subsystem of a larger system, which we describe briefly in this section and that Dürr, Goldstein, &amp; Zanghì 1992a: Section 5, discuss in some detail, together with the related notion of the effective wave function. 