What is "self"?

According to "An Invitation to Social Construction" written by Kenneth J. Gergen, the idea of "self" or "individual" originated in the philosophy of Enlightenment in 17 and 18 centuries. "self" or "individual" is in our each inner world, that is, mind. And out of this world, the physical world expands being opposed to the inner world. This ontology based on dualism seems to be very natural to us. However, should we think how our minds can give effects on our physical bodies? We can move our bodies' parts as we want to do. So what is the mechanism between mind and body? Once upon a time, although Rene Descartes thought such converting mind to body took place in our pineal bodies, the present brain science deny his hypothesis. But right from the beginning, Descartes did not explain the causal relationship between mind and body, and the explanation have not been proposed yet.
To avoid this dilemma of dualismic ontology, monism, that is the idea that there is only one world, only our mind world. According to this idea, only certain things are our experiences (this is the same idea of Descartes), and we live in our each complete closed private world, furthermore, the idea of existing the other people is a phantasy built by our minds. Investigating this idea, we arrived at solipsism, and this idea has no falsifiability.
The substitute for solipsism is materialism which is another monism. This idea affects on today's great development of science and technology. We have only one physical world constructed by materials, and our minds are only materialistic processes in our physical brains. However, what is "material"? The fact of existing materials is based on only our cognition. That is to say, materialism is equal to solipsism.
I wonder how we should think about "self" or "individual"?