Forming Diagnostic Opinions

Using an understanding of rhetoric as a method of communicative reasoning capable of providing grounds for conviction in those to whom it is addressed, this article argues that the formation of medical diagnoses shares a structure with Aristotle’s account of the rhetorical syllogism (the enthymeme). Here the argument itself (logos), together with characterological elements (ethos) and emotions (pathos), are welded together so that each affects the operation of the others. In the initial three sections of the paper, we contend, first, that diagnoses, as verdictive performatives, differ from scientific claims in being irreducibly personal and context-dependent; secondly, that they fit the structure of voluntary action as analysed by Aristotle and Aquinas; thirdly, that as practical syllogisms they differ from theoretical syllogisms, for example in taking effect in action, being ›addressed‹, and being intrinsically embedded in wider contexts of medical communication and practices. In the remaining sections we apply this account to textual evidence about diagnosis, drawing on work by the brain surgeon Henry Marsh. A rhetorical analysis of his observations on the formation of diagnostic opinions in situilluminates how moral, social and emotional features are fused with the cognitive aspects of medical judgement, making or marring how diagnoses and treatment are enacted. In other words, a philosophical- rhetorical account of diagnosis can help us to appreciate how medical diagnosis takes effect. We briefly conclude with some implications of our work for how diagnostic processes could in practice be better supported.