Consider now a different theory, call it "Theory II", which generates different S-structures from those of the two variants of Theory I. Theory II generates S-structures lacking empty categories - trace or PRO [...] Theory II is rather different in its properties from Theory I. For example, Theory II does not observe the projection principle; furthermore, it assigns θ-role to arguments that are not in θ-positions by devices quite different from those that are employed to relate operators such as "wh"-phrases to the variables they bind, to associate extraposed phrases with their antecedent, to interpret idioms, to deal with the properties of non-arguments that "inherit" features from their D-structure positions [...] all of these cases involving the rule Move-α in Theory I.
- Chomsky (1993), a pag.92