- thing-in-itself (Ding an sich: noumena). The real world is an authentic reality. This world is transcendent, so outside the grip of human experience.
- thing-for-us (Ding für uns: phenomena). The ideal world, on the other side, is a fictive reality. It is immanent and belongs to human experience. The distinction between “thing-in-itself”/ “thing-for-us” was introduced by Immanuel Kant.
- will (Wille). Arthur Schopenhauer believed has cracked the mystery behind the “thing-in-itself” identifying it, at a younger age, with the will. However, at maturity, he reconsidered his position, situating the “the will” between the “thing-in-itself” and the “thing-for-us”: restated, the will is the thing-in-itself as we can understood it.
- representation (Vorstellung). While the will has a controversial definition, depending on the two versions of “The World as Will and Representation”, the “representation” is what Kant named “thing-for-us”. It is a famous passage in this book, where Schopenhauer presents the relation between “will”/ “representation” as that between a weak lame (i.e. the representation) sitting on the shoulders of a strong blind (i.e. the will) [1969, Vol. II: 209]
Arthur Schopenhauer (1969): “The world as will and representation”, Dover Publications, Inc., Volume I & Volume II
Julian Young (2005): “Schopenhauer”, Routledge, Taylor & Francis Group