This book is inspired by the quest for the proper function of Aristotle's active intellect presented in De Anima 3.5. The urge to grasp its essence has always driven philosophy ahead. Nevertheless, the true nature of nous poiêtikos remains far from being grasped. I will not unravel it here either, I am afraid. My aim is far more modest: to get through the existing interpretations and try to systematise them somehow. To speak in a more poetic, metaphorical way: my aim is to shed some light upon the subject. I wish to explore the light metaphor from De Anima 3.5 on many levels. To avoid getting lost in Aristotle and Aristotelian interpretations, I have divided the readings of nous poiêtikos into mystical and rational ones.