The subject is a speaker of language. Language is the key link between all subjects; it is the core network of social existence. The subject is only a subject in language. Reality only exists through language. We can never escape the process of expression through language and what can be called subjectivization through language. No pure self-consciousness exists outside of language, even if the subject is simply sitting still and not speaking. Consciousness is only possible through the mediation of other consciousnesses. This is the central meaning of Lacan’s statement that the unconscious is structured like a language.

The subject in this sense of a speaker of language is fundamentally split. This is simply a way of referring to the impossibility of full and present self-consciousness or self-understanding. There will always be a gap between what one thinks one knows of oneself and what is hidden from view. The split or divided subject “is operative in all of the various ways in which we fail to identify ourselves, grasp ourselves, or coincide with ourselves” (Bracher, 113). This is also understood in terms of the split between the “I” who speaks and the contents of the statement that is spoken. In Lacanian terminology, the distinction is between the subject of enunciation -the I who speaks --and the subject of the enunciated, that is, the statement. There is the empty I that is the subject and there is the self that is part of concrete reality. Descartes said “I think therefore I am,” where “I think” supposedly designates a pure transcendental point of self-consciousness removed from the real world. But Kant (and also Lacan/ Žižek) would say that there is no way to say “I think” without attachment to the whole of reality. The “I” is “an empty, nonsubstantial logical variable” (Žižek, TN, 14) which is inherently inaccessible, is only purely possible, not concretely real. The I is a pure void, an empty void or frame only knowable through the predicates that make up the contents of what I think. I cannot acquire consciousness of myself except through the endless series of predicates and statements that fill out what the I thinks.
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At this level of the empty subject all subjects are equal. The subject emptied of all attributes is equal to any other such subject. Democratic equality can only work if each subject is willing to see itself and others at this level of bare abstraction. At the extreme, this implies that subjects must ultimately sacrifice what they feel is their most personal identifying trait. Equality is defined as the abstract equality of all such empty subjects.......Any feeling of confidence and fullness of self-determination is inherently false not because that is a bad way to be from a moral point of view but because it is so ontologically. “I am what I say I am” is thus an impossible statement. That is, as a statement it is only possible as a fantasy; you can only fantasize that you are what you say you are.
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Lacan/ Žižek, however, emphasize that the inherently split or divided nature of the subject in itself provides a place that allows escape from the grips of external reality. But escape is not a romantic notion in this case. The point is that the impossibility of a subject who is fully self-conscious, who fully knows himself, means that there is always part of the self that will be unknown and unattainable by the reach not only of the conscious ego but also of the symbolic order. As Freud already theorized, this part is referred to as the unconscious. One will always be an effect of outside, external, and unknowable elements. Language will traverse the subject in ways it will never fully grasp. But the subject retains an ultimate ability to resist that which traverses it. Resistance and self-invention and re-invention, however, involve acts of sacrifice and self-negation that most of us in our desire for stability can hardly endure.

Another way to phrase the function of language in the definition of the subject is to say that language is all we have to express ourselves (music and other forms of art should also be regarded as forms of language even if they are non-verbal). But we never feel that we completely succeed in expressing ourselves. Language seems never enough. This “never enough” is a left-over or a remainder that amounts to the void I was speaking about in the definition of the split subject. It is that void and lack that supplies us with our so-called escape from the oppression of the symbolic order. But again it is a void that has no insurance, no safety exits, nothing but insecurity although at the same time it offers possible liberation. Liberation comes in the form of revolutionary attempts, many of which fill will blissful energy and many of which end in failure. — http://kmcmahon.faculty.ku.edu//LacanZizeksum.html