We are also able to make abstract references to things that do not - and even cannot - exist. For example, there is a fundamental problem in mathematics and set theory called Russell's Paradox (after Bertrand Russell), which discusses the impossible (yet easily nameable) set of all sets that do not contain themselves. The problem with such a set is as follows: 

If the set contains itself, then it does not belong there.  But if the set does not contain itself, then it does belong there. Such a set cannot possibly exist, and yet we can define its properties in a description with ease.