Sentence:  Moreover , it is too readily forgotten that in the Republic what gave the initial impetus to Plato 's excursus into the construction of an imaginary commonwealth with its ruling-class communism of goods , wives , and children , was his quest for a canon for the proper ordering of the individual human <WSD>psyche</WSD> ; and it is to this problem that the Republic ultimately returns . 
Meaning of the word marked with <WSD> tags:  that which is responsible for one's thoughts and feelings; the seat of the faculty of reason