  Someone committed to a strong version of the medical model sees validating a diagnosis as understanding its underlying causal structure: a valid diagnosis rests on a biological process that can be identified by experiment and observation using the methods of the biological and cognitive sciences. Any approach to psychiatry that looks to science to validate its categories in this way must meet at least two conceptual challenges. First, there is a metaphysical challenge, which is that a concept of validity tied to the uncovering of neurobiological processes seemingly adopts a realist philosophical position, thinking that science can tell us how the world is really put together. Many philosophers think that realism is a wholly unnecessary and unwarranted metaphysical commitment, and that all science can really tell us about is a set of relationships among the data. These relationships let us make predictions and exert some control over nature, but do not tell us what is really out there. Philosophical disputes over realism have often centred on the notion of the unobservable. In psychiatry the situation is different; theories and constructs refer to latent variables rather than unobservables.  Schizophrenia, as a construct, is not a further type of stuff that lies behind the appearances and explains them: it is constituted by visible phenomena. We are not looking for an unobservable, but asking which observable phenomena (neurological, genetic, developmental, social) explain the symptoms. If the strong conception of the medical model is philosophically realist, it is so in light of its commitment to causes, not to unobservables.  We may look for a hidden variable that explains the observable phenomena, or for some other hypothetical underlying cause. Gold and Gold (2014, p.163–66) for example, suggest that many delusions and other psychotic symptoms may reflect a malfunctioning “Suspicion System” that has evolved to detect and monitor social threats. If such a system exists it is not directly observable, but its components should be open to observation and manipulation via the standard methods of the cognitive sciences. Psychiatry, like many sciences, deals in hidden variables or hypothetical causal structures, but it is not helpful to think of these as unobservable in the traditional sense.  