From antiquity, heat and moisture were key concepts in European understandings of the generation of life; philosophers and physicians used them to explain ageing, dying, disease and sex difference. This chapter sketches the dominance of heat, and subsidiary role of moisture, in cosmological and physiological theories from pre-Socratic philosophers and Hippocratic physicians, through Aristotle and Galen, about the production and quality of seed and the formation of an embryo. When medieval Arabic scholars reformulated Galenic medicine, they developed the concept of radical moisture and put it on par with innate heat. By the fifteenth century, radical moisture, as an epistemic category, had mitigated the stark divisions of the Aristotelian view of generation by fostering a stronger interest in the materiality of generation; this led to a new understanding of active matter, a notion that thrived in alchemical medicine. In the sixteenth and seventeenth centuries, categories of innate heat and radical moisture were rethought and changed theories of generation, especially concerning semen. By the end of the seventeenth century, although the categories of innate heat and radical moisture were no longer perceived as useful, attempts to rethink radical moisture were important to new theories of generation that emphasized the ovum.