I know who I am. Is this an accomplishment or a tautology? On the "modern" view, the self (who I am) has perfect access to itself. It is given in experience as its center; experience goes on around it. The subject, on this view, has certain essential characteristics that are given in advance of experience (a priori). It does not learn what (who) it is by experience. Rather, what experience is—human experience, at least—depends on these pre-given characteristics of the subject. Who I am is not an empirical question. It is what I know before I begin my inquiries.
On the modern view of science, then, the scientific subject (the subject of scientific knowledge) is given in advance of any particular study. The study does not draw its own subjectivity into question. With postmodernism, however, this changes. The point from which the world is studied becomes part of the question we put to the world. Just as we cannot simply assert, without argument, what is the case, we cannot simply assume who we are. Nor, perhaps more importantly, can we assume that we will remain who we are during or after the study.
The study itself will change us. And we will change in any case, independent of the study. The self does not stand in the middle of life watching it happen all around; it is merely a part of life, part of the process. So, on this view, your sense of self is an accomplishment; it is not an entitlement. Moreover, it is a collective accomplishment. We are not alone in being ourselves. Others have a constitutive influence on our identity.
These notions—subjectivity, identity, self—go together, and with another: agency. In the past (before, say, 1968), they marked the limits of appropriate inquiry. "I am I" was not open to doubt in decent company. We were free agents, we had our rights. Under "postmodern conditions", "I am I" is an entirely empty, and somewhat quaint notion. It is like the prisoner who says to the guard, "But I am different. I am innocent." "Sure you are," says the guard. "Sure you are."*
The interesting thing here is that whether or not the self is "centered" is itself either an empirical question or not. If you are a modernist, you oppose the idea of even raising it, except in the privacy of your mind, or perhaps in the confidence of your "personal" acquaintances. It is none of our business. It is not a public concern. The postmodernist, however, takes your identity to be very much a public matter. "Yes, it's fucking political!" declares Skunk Anansie. No, it's fucking not, says the modernist. (I sometimes take that line myself.)
This is no doubt one reason that postmodernism is a fundamentally contested notion. It cannot bring itself up without controversy. It isn't sure of itself.
*I can't at the moment remember what novel this scene is from. Camus' The Stranger? Kafka? Dostoevsky? If anyone recognizes it, please let me know.