Strange Loops
My favourite paradox...
A strange loop is a cyclic structure that goes through
several levels in a hierarchical system. It arises when, by moving only
upwards or downwards through the system, one finds oneself back where
one started. Strange loops may involve self-reference and paradox.
Below is an example of a Strange Loop: Drawing Hands is a
lithograph by the Dutch artist M. C. Escher first printed in January
1948. It depicts a sheet of paper, out of which two hands rise, in the
paradoxical act of drawing one another into existence. This is one of
the most obvious examples of Escher's common use of 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.
Read more about Strange Loops on
Wikipedia