Brain is a big clump of neurons. Though each of the neurons behaves independently and discretely, the mass of neurons looks to work with continuous value in group. An important feature observed in it is the symbolic thinking that is specific for human brain. The symbolic thinking has features of discrete, logical and conscious that are not seen in current brain understanding and in conventional neural network models, and the features are essential for making human intelligent. In this talk, we overview the history of researches that investigated human brain intelligence, and introduce some researches that may lead to the symbolic thinking study in future.