Hierarchical Systems Theory (HST) has two parts:
1. In the first part (http://mind-phronesis.co.uk/category/part-1), HST demonstrates how phenomenal experience is an emergent property. It describes hierarchical categories of evolutionary development of mind, and explains why each category emerges and possesses its particularly unique behavioural and physiological characteristics.
Unique to HST, is its ability to extrapolate the next evolutionary stage of human mind development.
2. In the second part (http://mind-phronesis.co.uk/category/part-3), limitations of HST (which are that it is merely a reductive explanation of phenomenal experience), demand the exploration of the concept 'nomenal experience'. Its author suggests a state vector solution.