Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A differential diagnosis (sometimes abbreviated DDx, ddx, DD, or ) is a systematic method used to identify unknowns. This method, essentially a process of elimination, is used by taxonomists to identify living organisms, and by physicians, physician assistants, and other trained medical professionals to diagnose the specific disease in a patient. Not all medical diagnoses are differential ones: some diagnoses merely name a set of signs and symptoms that may have more than one possible cause, and some diagnoses are based on intuition or estimations of likelihood.