In the tradition of generative grammar, two different definitions of "subject" have been proposed: we name the first one "structural", since it is based on the constituent structure of the sentence, and the second one "thematic", because it makes use of the concept of thematic role. We argue that both definitions are necessary, since they deal with two different entities, i.e. the structural subject and the thematic subject, which are to be kept distinct. In particular, we show that opacity phenomena are induced by the "thematic" subject, and not by the "structural" one (in showing this, we make use of the notion of "Complete Functional Complex" recently proposed by Chomsky); this kind of analysis allows us to dispense with the so-called "i-within-i condition", which appears theoretically unsound. Some cases of behavior of anaphors which seem to run against our proposal are also discussed, and it is shown that they can be accounted for on the basis of independent principles.