 This leaves unaddressed the issue of how we determine what normal function is. Wachbroit (1994) argues that when we say that an organ is normal, we employ a biomedical concept of normality that is an idealized description of a component of a biological system in an unperturbed state that may never be attained in actual systems. Boorse (1977, 1997) insists that the notion of normality in biomedical concepts is statistical — how things usually are in a reference class, but this view faces the problem of specifying the reference classes in an informative way. But given the amount of variation within a species, it will always be hard to find reference classes which share a design. As Ereshefsky (2009) puts it, Boorse assumes that statistical normality coincides with the kind of normality that medicine cares about, but this looks wrong. Wachbroit (1994, 588) argues convincingly that the role of normality in physiology is like the role that pure states or ideal entities play in physical theories.