The kernel of human consciousness is self-consciousness. We in this article explore the issue about what subjective-self is composed of. A distinctive feature of "I" is uniqueness or un-duplicableness. By critically reviewing a few thought experiments, we reason that subjective-self must be composed of more than a blob of brain cells which are sophisticated enough to think reflexively and a bundle of memories and perceptions. There must be a third constituent for the existence of "I". We put forward two hypotheses for the third constituent. One is that it is something like dark matters or dark energy. Another is that it is something in the higher dimensional space. A person would lose his self-consciousness after death since the death takes away the two necessary constituents of subjective-self, his brain and memories. Where the third constituent would go after death remains a mystery.