Sigmund Freud's theory of the structure of the human psyche constitutes one of the most recent variations on this three-fold theme. Freud's three psychic components, the Id, Ego, and Superego, can be aligned upon a vertical up/down axis (as shown on the left) as an aid to understanding. Still, Freud's most influential student, Carl Jung, decided that there were actually four components in the psyche (as shown on the right),²² only one of which was included by Freud. Jung's four components—The Ego, The Mask, The Shadow, and the Soul—can be put into a horizontal cross-like structure similar to the four Cardinal Directions. Jung believed that these four components had a social origin: a four-fold familial relation that Jung calls "the basic schema of cross-cousin marriage", which (according to Jung) was assimilated into the human psyche during the long history of humanity.