We saw in class that the predicate given in this forum title creates some problems for set theory. This paradox was described in a famous letter written just about 1 century ago. Bertrand Russell wrote the letter. Who received it? Why was it important?
X is not an element of X
Russell wrote the letter to Gottlob Frege on June 16, 1902 and it was important because it pointed out the inconsistencies in the axioms Frege used to support his theory of the foundation of mathematics.