Hickam's dictum is a counterargument to the use of Occam's razor in the medical profession. The principle is commonly stated: "Patients can have as many diseases as they damn well please". The principle is attributed to John Hickam, MD.
When discussing Occam's razor in contemporary medicine, doctors and philosophers of medicine speak of diagnostic parsimony. Diagnostic parsimony advocates that when diagnosing a given injury, ailment, illness, or disease a doctor should strive to look for the fewest possible causes that will account for all the symptoms. However, this principle has very important limits in medical practice. The actual process that occurs when diagnosing a patient is a continuous flow of hypothesis and testing of that hypothesis, then modifying the hypothesis, and so on. In the context of this method, the principle of Hickam's dictum asserts that at no stage should a particular diagnosis be excluded solely because it doesn't appear to fit the principle of Occam's razor. The principle of Occam's razor, or parsimony, does not demand that the diagnostician necessarily opt for the simplest explanation, but instead guides the medical practitioner to seek explanations, without unnecessary additional assumptions, which are capable of accounting for all relevant evidence.
A key reason for using Hickam's dictum as a limiting principle to that of Occam's razor is, it is often statistically more likely that a patient has several common diseases, rather than having a single rarer disease which explains the myriad of their symptoms. Another key reason is that, independently of statistical likelihood, some patients do in fact turn out to have multiple diseases. In such cases multiple categories of diagnosis may indeed have independent causes rather than a single source, i.e. may be due to separate events or combinations of events to which the patient may have been subjected or exposed. Thus Hickam's dictum provides physicians with a counterbalancing principle to the unfettered use of Occam's razor in diagnosis.
An example of the utility of Hickam's dictum is Saint's Triad of hiatus hernia, gallbladder disease and diverticulosis. C. F. M. Saint was a South African surgeon. His triad has no known pathophysiologcal relationship nullifying the usefulness of Occam's razor. Hickam's dictum is similar in principle to Willian of Chatham's anti razor.