This passage, from the section De Transformatis in the eleventh book of Isidore's Etymologies, reveals three ways in which medieval learning explained shapeshifting: it could derive from moral causes (miscreants transformed due to crimes), from magic (the reference to charms) or it could be caused by natural action (as with poisonous herbs and decay). These three causes are paralleled in other thinkers, such as Arnulf of Orléans, who thought that metamorphoses were caused by natural, artificial, magical, or spiritual reasons.

Arnulf and Isidore are characteristic of a frequent medieval worldview which could be described as conceiving the different aspects of the universal order as closely interconnected with each other (and as stemming from God). This view logically considered the natural aspects of order as more fluid than a modern, post-Linnean view, and was dominant in the whole medieval period, or at least until the intellectual transformations brought about through neo-Aristotelian thought from the central Middle Ages onwards. In such a medieval mode of thought the change expressed through metamorphosis was an integral part of the created world, even if frequently circumscribed to remote geographical corners, or the effect of the work of specific agents, be they divine or demonic.

It is not difficult to imagine why the medieval multidimensional ideas about causation, nature, and transformation of beings were often also concerned with shapeshifters: these appear almost as embodiments of this fluid, interconnected image of reality. While our focus in this book is in its literary manifestations in Northern European contexts, the multitude of werewolves, swan-maidens, bear- and boar-men that appear in medieval narratives should not be thought as remnants of ‘primitive’ or ‘folk’ mentalities: shapeshifters were a core element of medieval cultures and were a concern even for the clerical, learned elite.