  Medieval natural philosophy included both the rich commentary tradition on Aristotle's Physics, as well as treatises and commentaries on Aristotle's other writings about the natural world: On the Heavens, On Generation and Corruption, De anima, the short treatises on animate powers known as the ‘Parva Naturalia’, and the works on the history, parts, and generation of animals. Buridan wrote commentaries on nearly all of these texts. Like his contemporaries, he understood the speculative sub-disciplines of physics as an orderly progression of teachings. Thus, demonstration in psychology was thought to borrow its principles or starting points from physics, the higher science to which it is subordinated, with its conclusions in turn supplying principles for demonstrations in the more specialized studies of sensation and animal locomotion. The ordering metaphor is never far from Buridan's mind. He remarks to readers of his commentary on Aristotle's De motibus animalium that in this book, “we descend to the different species of motion in particular, e.g., to the fact that some animals fly, others swim, and so on” (DMA I: 535)