  Some theorists would claim that (21) is true only if John has a first-person desire, that is, a desire that he would naturally express with ‘I want to eat a large slice of pizza’. Some of these theorists would attribute the seemingly mandatory first-person reading of (21) to the presence of a silent, pronoun-like expression that serves as a subject for the embedded infinitival phrase ‘to eat a slice of pizza’. This sort of pronominal expression is often called ‘PRO’. For discussion, see Dowty 1985, Chierchia 1989, and Cappelen and Dever 2013.