Such languages as Spanish and Italian differ from French and English in a distinct parameter, namely, what has been called the "pro-drop parameter", which entails a variety of consequences […] that is, in these languages, not only "weather verbs" such as "rain" but also verbs that have subjects with a definite θ-role may appear at surface structure with no NP subject.
- Chomsky (1993), a pag.28

Assume "R" to be, in effect, a rule of Affix-movement. If "R" applies in the PF-component, then AGR governs the subject position at S-structure and at LF [...] Suppose that we now take the pro-drop parameter to be (12): (12) "R" may apply in the syntax. The pro-drop languages accept this option [...] The non-pro-drop languages reject the option (12).
- Chomsky (1993), a pag.256-257