Fig. 4. The 20th century: compliance with Bernard's paradigm and early skepticism. Hans Horst Meyer (A ) and Charles Ernest Overton (B ), working within the unified paradigm, formulated the eponymous rule at the turn of the century. Although ubiquitously quoted today, the Meyer-Overton “rule” had only a limited effect on theories of anesthetic action until late in the 20th century. To the best of my knowledge, Thomas C. Butler (C ) was the first to publicly question the unified paradigm2but, judging by the trajectory of research in the decades following this publication, his critique had little effect. The images of Hans Horst Meyer and Charles Ernest Overton are courtesy of the Wood Library-Museum of Anesthesiology, Park Ridge, Illinois. HHM is pictured many years after his well-known publication of 1899. The picture of Thomas C. Butler was kindly provided by Todd Dawson from the Office of Medical Alumni, University of North Carolina, Chapel Hill, North Carolina.