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.A tangled hierarchy is a hierarchical consciousness system in which a strange loop appears.