Invention: Once a draftsman has mastered the ability to reproduce the object before him, he begins to "delineate upon some surface, a subject from memory or from a book...or from [his] imagination (fantasia), which is commonly called inventar or to debujar con fantasia" in order to "bring to light ideas from [the artists'] understanding which were conceived by means of reason, discourse, observations and precepts."
The key to invention is knowledge, practical and theoretical/philosophical.