The introduction does the following: it lays out the scope and argument of the book; it explains different senses of free will, a broad sense, which does not presuppose the ability to do otherwise, and a narrow sense, which does presuppose it; it presents broad definitions of historiographical labels: intellectualism and voluntarism, which respectively refer to theories that explain free agency mainly with reference to the intellect or the will; it summarizes the commonly accepted narrative of the fall of the angels, of which medieval thinkers discuss particular aspects in connection to free will; and it presents a brief chapter outline.