Function words are commonly considered to be a small and closed class of words in which each element is associated with a specific and fixed logical meaning. Unfortunately, this is not always true as witnessed by negation: on the one hand, negation does reverse the truth-value conditions of a proposition, and the other hand, it does not, realizing what is called Expletive Negation. This chapter aims to investigate whether a word that is established on the basis of its function can be ambiguous by discussing the role of the syntactic derivation in some instances of so-called Expletive Negation clauses, a case in which negation seems to lose its capacity to deny the proposition associated with its sentence. Both a theoretical and an experimental approach has been adopted.