[…] non-argument "it" (as in "it is certain that John will win") or existential "there" (as in "there are believed to be unicorns in the garden"), terms which assume no θ-role.
- Chomsky (1993), a pag.35

Non-arguments include other non-NP categories as well as NPs that are non-referential: impersonal "it", existential "there", perhaps idiom chunks, and analogues in other languages. Since NP-trace transmits its θ-role (if any), I will take trace to be a non-argument if it is not a variable.
- Chomsky (1993), a pag.101

The pronoun "it" can be a true argument ("it is on the table"), a quasi-argument ("it is raining"), or a non-argument ("it seems that John is here").
- Chomsky (1993), a pag.325