The paper examines the epistemological reactions of the eighteenth-century scientific world to the regeneration of the polyp, discovered by Abraham Trembley in the early 1740s. I have coined one of these reactions, the ‘rule of the exception’, adopted by Antoine Ferchault de Réaumur and the Réaumurians to illustrate the place and impact of the new discovery. In 1742, Réaumur signaled the exceptions that the polyp brought to the "general rules", that is, to the laws of life. There were exceptions to some laws, such as mating, which characterized only some classes of organisms and not all beings. Later, in the 1760s, Charles Bonnet outlined several rules that combined the heuristic power of the scientist with the diversity of models in nature. The paper unified these rules, called Bonnet's principle, which, along with the rule of exception, reorganized the epistemological space of the life sciences, far from the traditional antagonism between preformation and epigenesis.