How Theology, Imagination, and the Spirit of Inquiry Shaped Natural Philosophy in the Late Middle Ages

Historians of science have long debated historiographical issues and have even come to regard some of them as rather passe. The issues I shall consider will be applied to the late Middle Ages and, where necessary, extended to the early modern period, or, as I shall refer to it, the Scientific Revolution. In the broad range of medieval science and natural philosophy, I shall focus almost exclusively on natural philosophy, because, as we shall see, natural philosophers posed questions that probed "into all aspects of the world: nature, the supernatural, and an imaginary world of the hypothetical and possible".1 It included bits and pieces of virtually all of the contemporary sciences, as well as thoughts and ideas that would be appropriate to sciences that only came into being in the modern era. It was not until the sciences of astronomy, optics, and mechanics - the exact sciences, known as the "middle sciences" to Aristotle and his followers in the late Middle Ages - became fully integrated with natural philosophy in the seventeenth century that early modern science emerged. Although the process of integration began in the Middle Ages, it accelerated rapidly in the seventeenth century. The importance of the union between the exact sciences and natural philosophy was truly significant. Other societies that at one time had well-developed mathematics and astronomy, but failed to generate, and maintain, a well-developed natural philosophy, eventually saw their mathematical sciences fade away. A prime example is the civilization of Islam.2 In what follows, I shall focus on two basic issues that are most relevant to natural philosophy and should be of great interest to historians of science. I shall first consider the influence of theology and religion on natural philosophy and the exact sciences; and then investigate a topic that, although connected to the first theme, emphasizes the role of reason, imagination, and counterfactuals, which taken together produced a spirit of inquiry that encouraged the investigation of nature and the development of science. Because natural philosophy is the focus of my essay, I shall now present a brief summary of the highlights of medieval natural philosophy.NATURAL PHILOSOPHY IN THE LATE MIDDLE AGESDuring the Middle Ages, natural philosophy was solidly based upon Aristotle's "natural books [libri naturales]", which included Physics, On the heavens, On generation and corruption, Meteorology, On the soul, and The short physical treatises (Parva naturalia). Since these works treated different aspects of the physical cosmos, natural philosophy ranged over all aspects of the physical world that involved change. For most scholastic natural philosophers, the subject matter of natural philosophy was 'mobile being [ens mobile]', a term that Thomas Aquinas used. 'Mobile being' rather than 'mobile body' was used because it embraced the motion of both material and immaterial substances. Natural philosophy was concerned with change as manifested by things in some kind of motion. As an anonymous fourteenth-century author expressed it, "the whole of movable being is the proper subject of natural philosophy".3 Most of the subjects discussed were drawn from the broad range of Aristotle's natural philosophy.Medieval natural philosophy was presented in three quite different literary formats. The first, commonly in use through most of the thirteenth century, was the commentary. In this format, whichever of Aristotle's natural books was the subject of the commentary, the author would present a few lines of Aristotle's text and then explain them while also commenting on them. He would then do the same for the text immediately following, and move through the entire treatise in the same fashion, paragraph after paragraph until the commentary was completed. The second, and most popular, way of proceeding was by way of a series of questions, each of which usually began with the word 'whether [utrum]\ In the Physics, we could readily meet a question such as "Whether the existence of a vacuum is possible"; in On the heavens, we might confront the question "Whether the whole earth is habitable"; from the Meteorology, a question on visual rays would be fairly common, such as "Whether every visual ray is refracted in meeting a denser or rarer medium"; and in questions on On generation and corruption, it would be frequently asked "Whether elements remain [or persist] formally in a compound [or mixed] body". …