In medieval and early modern natural philosophy, very few issues were as controversial as the nature of the elements. From the thirteenth up until the sixteenth century, European thinkers discussed this problem with growing interest. Defining the nature of the elements was key to deciphering the very structure of the universe and the essence of things. Along with five primary texts, here edited for the first time, this book discusses one of the most original contributions to this debate, that of Renaissance philosopher Pietro Pomponazzi (d. 1525). Pomponazzi’s account, developed in university lectures, holds significance for two reasons. First, it provides a thorough description of the most influential doctrines on the elements presented by medieval scholars, opening a window onto three hundred years of prior discussions on the topic. Second, Pomponazzi also develops his own views on the issue, explicitly defining them as ‘heretical’ to emphasise his departure from all opinions expressed before him.