Sentence:  `` History has this in common with every other science : that the historian is not allowed to claim any single <WSD>piece</WSD> of knowledge , except where he can justify his claim by exhibiting to himself in the first place , and secondly to any one else who is both able and willing to follow his demonstration , the grounds upon which it is based . 
Meaning of the word marked with <WSD> tags:  an item that is an instance of some type;