 However, how we ascertain what is pathological remains to be determined. Not all diagnoses can be tied to a break between normal and abnormal functioning of an underlying mechanism, such as a failure of the kidneys to conserve electrolytes. Nor can we always discover some other abnormality, such as the elevated levels of helicobacter pylori bacteria that have been found to be causally implicated in stomach ulcers (discussed in detail by Thagard 1999). Some conditions, such as hypertension, involve cutting between normal and pathological parts of a continuous variation, even in the absence of clear underlying malfunctions that separate the populations. The Boorsian tradition has tried to deal with the problem of variation by tying assessments of function and malfunction to reference classes, which Boorse (1977) treated as natural classes of organisms that share a uniform blueprint. Kingma (2007, 2010) has recently argued that reference classes cannot be established without normative judgements, contra Boorse, who takes them to be objectively discoverable parts of the natural order. Kingma contends that Boorse’s account of function needs to capture not only the qualitative causal contribution made by a system to overall functioning, but also the quantitative features of its contribution: a healthy heart is not just a pump, but a pump that works at a given rate. In addition, a system must be capable of working in a variety of situations, including rare ones that require a physiological response to a crisis. Kingma argues that Boorse’s biostatistical theory cannot capture statistically unusual yet functional situations, and concludes that we need to appeal to situation-specific functions. (Cf. Canguilhem (1991 p. 196), who argued that disease is only abnormal relative to a clearly defined context.)