The vitality of physiology is currently under debate. Some say that physiology is a dying discipline in the era of molecular medicine and systems biology, whereas others claim that physiology remains a key biological and medical discipline, due in part to its integrative nature. In this conceptual review, we argue that any assessment of the vitality of physiology depends heavily on the definition of this discipline adopted. We examine two main conceptions of physiology, one focusing on its object (what physiology is about), and the other on the methods used (how physiologists study the biological reality). We contend that physiology no longer encompasses all biological disciplines and may no longer be the only synoptic biological science. However, far from indicating a sterility of this discipline, this situation should drive physiology to re-invent its relationship with these other biological domains.