A
strange loop arises when, by moving only upwards or downwards through a hierarchical system, one finds oneself back to where one started. Strange loops may involve
self-reference and
paradox. The concept of a strange loop was proposed and extensively discussed by
Douglas Hofstadter in
Gödel, Escher, Bach, and is further elaborated in Hofstadter's book
I Am a Strange Loop, published in 2007.