In this paper I describe the context and goals of Francisco Vallés' In IV librum Meteorologicorum commentaria (1558). Vallés' work stands as a landmark because it interprets a work of Aristotle's natural philosophy specifically for medical doctors and medical theory. Vallés' commentary is representative of new understandings of Galenic-Hippocratic medi-cine that emerged as a result of expanding textual knowledge. These approaches are evident in a number of sixteenth-century commentaries on Meteorologica IV; in particular the works of Pietro Pomponazzi, Lodovico Boccadiferro, Jacob Schegk, and Francesco Vimercati. Vallés' conviction that Meteorologica IV is relevant to medical knowledge depends on his understanding of Aristotle's theory of homeomerous substances and their relation to composite substances. The application of Meteorologica IV to medical topics became commonplace in the following years, and this Aristotelian book became widely known as a bridge between natural philosophy and medicine.