In this treatise, Claude Bernard argues that biology and medicine are subject to the same sort of scientific scrutiny as are other sciences (not yet a universally accepted notion); he then sets forth a basic outline of the approach to scientific inquiry in biology and medicine and warns of the intellectual traps into which a researcher may readily fall. It is an historical landmark, marking the transition of medicine from an artisanal craft to a practice grounded in science. It is perhaps most striking, however, for its prescience. The common errors of scientific design and inference that trouble us in the 2000s were nicely described by Prof. Bernard 150 years earlier.