In linguistics, the empty category principle (ECP) was proposed in Noam Chomsky's syntactic framework of government and binding theory.
The ECP is supposed to be a universal syntactic constraint that requires certain types of empty categories, namely traces, to be properly governed.
The ECP has been revised many times and is now a central part of government and binding theory.
The chief function of the ECP is to place constraints on the movement of categories by the rule of move α; it effectively allows a tree structure to "remember" what has happened at earlier stages of a derivation, and it can be seen as GB's version of the older derivational constraints.
The intermediate traces must be deleted because they cannot be properly governed; theta-government is impossible because of the position they occupy, Spec-CP; the only possible antecedent-governor might be an overt NP (a wh-word), but the Minimality Condition would always be violated because of the tensed I (which must be present in all matrix clauses), the tensed I would c-command the intermediate trace but it would not c-command the wh-word.