Do you notice that I am asking "what constitutes the self" instead of "what is the self" here? Self is such an abstract concept that we can only understand it either through its relationship with the non-self world or through what it is made of.
I see self as the inner world trapped inside our body which mirrors the external physical world. We perceive the physical objective reality through senses and turns it into subjective reality in our brain. The interesting thing about the inner self world is that it is constantly interacting with the external world through information exchange and keeps evolving. So, self includes both the instrument of interacting with external world (how our mind process information and make decisions), and the result of it (our thoughts, values, beliefs, emotions, behaviors, etc.).
Humans has spent long time and made huge progress in understanding the fundamental constituents and motions of matter, planets and biological bodies. However, the journey of probing into our own mind has been fairly recent and still a long way to go. At school, we often learn about science, social science and humanities, but rarely we do spend enough time into learning human mind and ourselves despite the significant role of self-knowledge in shaping our lives. In fact, similar to the external physical world, the self-world has many layers, complex structures and inter-relationships. I am going to explore them in detail in my later posts.