This book is original, in the sense that the author, Steven A. Epstein, has come up with his own question, picked sensible sources to answer it and set out his results accordingly. When and how did medieval people ‘discover’ nature? Definitions are essential, and Epstein starts with one from William of Conches (c.1130): natura est vis quaedam rebus insita, similia de similibus operans (‘nature is a certain power grafted into things, making like from like’). This introduces three chapters (of the total five) on medieval genetic theory. The theorists were intellectuals, reluctant to consult rustici about experience, working almost entirely in libraries—albeit libraries enriched, after the twelfth century, by translations of Greek and Arabic scientific works. Experience nevertheless did deliver a few shafts of light. Inveterate skills of grafting plants had taught what could and could not be grafted, and that the result did not reproduce ‘like from like’. It was the same with the mule, hybrid between male horse and female donkey (or occasionally vice versa). The mule gets a chapter to itself, rehearsing speculations on why the mule was barren. Genetic mutation was a notion just round the corner for falconry specialists, speculating how differing breeds had developed. Only one writer, a former estate bailiff who had become a Dominican friar, openly mentions conversing with peasants about the choice of the most productive seeds. Other empirical data came from the common stock. Why do tall parents often have tall sons, but one-legged parents never have one-legged sons? Why and how does a child come to look like its parent, and which parent? (A woman who wants a beautiful child should look at something beautiful during intercourse: a measure established by Jacob in Genesis 31:27). Greeks had long known that characteristics could hop a generation, and this came in handy for a white woman, in a paradigmatic case in Quintilian, challenged to say how she had had a black child. Besides genetic ‘nature’, there was also the question of nature in economics, and in the agency of disasters. Is property natural? If so, were some Greeks right to think this applied to the ownership of slaves? Is the use of money natural? If so, are there ‘unnatural’ uses, especially usury? As to disasters, the subject of Chapter Five, were earthquakes, avalanches, town fires, storms, floods, and plague sent by God or another supernatural agency, or just by nature—this last alternative implied by the appearance of marine insurance contracts in fourteenth-century Genoa. The author is an established authority on Genoa (whence, partly, his interest in mules, invaluable in that steep city), and gives us another thought-provoking feature from a Genoese chronicle covering the years 1311–1435. Before 1348 it records a wide variety of disasters; afterwards, only recurrences of plague, as if that was now the only disaster that mattered.