Sentence:  In any inquiry into the way in which great literature affects the emotions , particularly with respect to the sense of harmony , or relief of tension , or sense of `` a transformed inner <WSD>nature</WSD> '' which may occur , a most careful exploration of the particular feature of the experience which produces the effect would be required . 
Meaning of the word marked with <WSD> tags:  the essential qualities or characteristics by which something is recognized