The debate about the relation between psychiatric disorder and neural defect has produced different argumentative strategies for and against the identification of these two phenomena. I'll coin these strategies as an ontological strategy, an extensional strategy, and an intensional strategy, on which I will focus in this article.
The first, ontological strategy takes the long road over a detailed characterization of the nature of psychiatric disorders and of neural defects. It then goes on to argue for a relation between these two ontological kinds. One anti-reductionist proposition of an ontological strategy can be found in Stir. He provides a sketch of a theory of the nature of psychiatric diseases, claiming inter alia that psychiatric diseases are social constructions and intrinsically normative. He goes on to infer a non-reducibility thesis from these more or less ontological characterizations: “if the boundary between normality and mental disorder is a social construction such that the question of whether a certain kind of behaviour is a disorder can only be judged against the background of this very convention, then the “disorderness” of a condition cannot be found on—and hence not be reduced to—the neuronal level”. While Stier's argument fluctuates between an epistemic and ontological non-reducibility thesis, I take him to be talking about the nature of psychiatric diseases and thus about an ontological issue foremost.
The second, extensional strategy investigates the phenomena that fall into both categories in order to relate the categories. In principle one would have to identify all psychiatric diseases and their pathways in the brain. If one could identify at least one neural causal pathway for each psychiatric disease, all psychiatric diseases could be assumed to be brain diseases. Alternatively one could use a falsificatory strategy and look for one psychiatric disease, which does not have a neural causal pathway. Obviously both are only in principle and not practically viable options. Universally quantified statements and claims of non-existence are well-nigh impossible to prove. Typically, the extensional method is applied in exemplary research projects. Proponents of the identity or reducibility of psychiatric disease to neural phenomena try to show that a certain psychiatric disease can be explained with reference to neural phenomena. Opponents of reducibility try to identify psychiatric diseases for which an explanation via some neural, causal pathway is improbable.

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