Because mythology is rich with doubles, twins, and twin aspects of personalities, this motif features prominently in Maps of Meaning, which studies mythopoesis, or the making of myths. From the onset, the book mentions sets of twins and brothers, such as Osiris and Seth of Egyptian mythology, gods respectively of order and disorder, as well as Christ and Satan of the Christian mythos. In Chapter 5, Peterson discusses the mythic “‘hostile brothers’—Spenta Mainyu and Angra Mainyu […] Gilgamesh and Enkidu, Cain and Abel” (319) Spenta Mainyu and Angra Mainyu are brothers created by the father-God Ahura Mazda; the former is benevolent and associated with light, while the latter is malevolent and associated with darkness. In Sumerian mythology, the wild Enkidu is the counterpart to the orderly Gilgamesh. In the Bible, Cain kills his gentle brother Abel; they are the first two offspring of the primordial couple Adam and Eve. A single figure or personality type can mirror duality as well, such as the creative and destructive Great Mother, benevolent Mary, and terrifying Kali. Furthermore, even seemingly opposite human behavior and stances are often two sides of the same kind, such as the fascist and the decadent.

The book explores values, qualities, and substances in their double aspect.