We present a case to illustrate the importance of emphasizing elementary physiology to deduce the basis for the acute onset of polyuria and hypernatremia. An imaginary consultation with Professor McCance is utilized to illustrate how a clinician-physiologist would have explained why these abnormalities developed and how they should have been treated. His approach began with a consideration of the most impressive abnormality. His analysis relied heavily on deductions and the anticipation of the expected responses to a stimulus in quantitative terms. The goals of therapy became evident after he performed mass balance calculations. Professor McCance would not understand why modern clinicians abandoned this form of analysis.