[...] we can distinguish two sorts of clitics: simple and special. The former we take to be elements corresponding to members of other syntactic classes; they appear in positions relative to the rest of the clause where they would (or at least could) be put by the normal rules of the syntax, but they happen to form a phonological unit with some other part of the structure.
- Anderson (2004), Pag. 27

A simple clitic differs from other lexical items in lacking the prosodic status of "word": it has segmental, and possibly syllabic and even foot structure, but it is not a word.
- Anderson (2004), Pag. 28