Hickam's dictum is usually stated as "patients can have as many diseases as they damn (or darn) well please".
The importance of this dictum lies in it acting as a counterweight to Occam's razor, declaring that a patient's clinical presentation may be secondary to two or more pathologies, rather than the parsimony of a single condition explaining all the symptoms and signs as put forward in the eponymous razor. Indeed in many cases it is more probable that an individual has multiple morbidities underlying the presentation, instead of one unifying diagnosis.
Hickam's dictum has also been expressed in a slightly different way as Saint's triad.
References 2 and 3 below nicely illustrate the tensions between Hickam's dictum and Occam's razor in modern medicine 2,3.
History and etymology
This aphorism has been attributed to John Hickam (1914-1970) an American physician, who was Chair of the Department of Medicine at the University of Indiana 1.
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