Sentence:  It <WSD>is</WSD> to say rather , I believe , that he has brought to bear on the history , the traditions , and the lore of his region a critical , skeptical mind - the same mind which has made of him an inveterate experimenter in literary form and technique . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)