Sentence:  `` History has this in common with every other science : that the historian is not allowed to claim any single piece of knowledge , except where he can justify his claim by exhibiting to himself in the first place , and secondly to any one else who <WSD>is</WSD> both able and willing to follow his demonstration , the grounds upon which it is based . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)