Hickam's dictum is a counterargument to the use of Occam's razor in the medical profession. While Occam's razor suggests that the simplest explanation is the most likely (implying in medicine that diagnostician should assume a single cause for multiple symptoms), Hickam's dictum is commonly stated: "Patients can have as many diseases as they damn well please". The principle is attributed to John Hickam, MD. When he began saying this is uncertain.
A key reason for using Hickam's dictum as a limiting principle to that of Occam's razor is that it is often statistically more likely that a patient has several common diseases rather than having a single, rarer disease that explains their myriad of symptoms. Another key reason is that, independent 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.
The rhymed version is cited here, among others.