Sometimes things seem deceptively easy when we consider normal cases. With more unusual cases, the simplicity can disappear—as with self-reference. ‘Self-Reference: what is this chapter about?’ examines the concept of self-reference in various examples like: ‘This very sentence that I am now uttering is false’. It is possible for a name to refer to something of which it, itself, is part. Some of the most ancient paradoxes fall into this area of logic, such as the liar paradox. They challenge the assumption that every sentence is either true or false, but not both. What does this tell us about validity? Sentences can be true, false, both, but never neither.