Suppose that the derivation has reached the stage Σ, which we may take to be a set {SO1,...,SOn} of syntactic objects. One of the operations of Chl is a procedure that selects a lexical item LI from the numeration, reducing its index by 1, and introduces it into the derivation as SOn+1. Call the operation "Select". At the LF interface, Σ can be interpreted only if it consists of a single syntactic object. Clearly, then, Chl must include a second procedure that combines syntactic objects already formed [...] The simplest such operation takes a pair of syntactic objects (SOi, SOj) and replaces them by a new combined syntactic object SOij. Call this operation "Merge".
- Chomsky (1995), Pag. 226