By explaining how to sire multicolored horses, produce nuts without shells, and create an egg the size of a human head, Giambattista Della Porta's Natural Magic (1559) conveys a fascination with tricks and illusions that makes it a work difficult for historians of science to take seriously. Yet, according to William Eamon, it is in the "how-to" books written by medieval alchemists, magicians, and artisans that modern science has its roots.

These compilations of recipes on everything from parlor tricks through medical remedies to wool-dyeing fascinated medieval intellectuals because they promised access to esoteric "secrets of nature." To popular readers of the early modern era, they offered a hands-on, experimental approach to nature that made scholastic natural philosophy seem abstract and sterile.

In closely examining this rich but little-known source of literature, Eamon reveals that printing technology and popular culture had as great, if not stronger, an impact on early modern science as did the traditional academic disciplines.

Medieval interest in the secrets of nature was spurred in part by ancient works such as Pliny's Natural History. As medieval experimenters adapted ancient knowledge to their changing needs, they created their own books of secrets, which expressed the uncritical, empiricist approach of popular culture rather than the subtle argumentation of scholastic science.

The crude experimental methodology advanced by the "professors of secrets" became for the "new philosophers" of the seventeenth century a potent ideological weapon in the challenge of natural philosophy.