Physician: The patient is suffering either from disease X or else from disease Y, but there is no available test for distinguishing X from Y. Ttherefore, since there is an effective treatment for Y but no treatment for X, we must act on the assumption that the patient has a case of Y.
The physician's reasoning could be based on the principle that when success is possible only if a circumstance beyond one's control is favorable, then one's strategy must be based on the assumption that this circumstance is in fact favorable.