Uncertainty is the central, critical fact about medical reasoning. Patients cannot describe exactly what has happened to them or how they feel, doctors and nurses cannot tell exactly what they observe, laboratories report results only with some degree of error, physiologists do not understand precisely how the human body works, medical researchers cannot precisely characterize how diseases alter the normal functioning of the body, pharmacologists do not fully understand the mechanisms accounting for the effectiveness of drugs, and no one can precisely determine one’s prognosis.

Nevertheless, every healthcare professional must make important decisions about testing and treatments and despite all the uncertainties about the bases for those decisions, the decisions themselves must be definitive. Hence, physicians generally seek means to relieve these uncertainties, e.g., one typical approach is temporizing: putting off the fi¬nal decision and hoping that additional information will evolve to make the decision more straightforward. Another approach is buying additional information by performing less risky or costly tests that help to reduce the uncertainties in the more critical decision.