Sentence:  Whitehead contends that the human way of understanding existence as a unity of interlocking and interdependent processes which constitute each other and which cause each other to be and not to be <WSD>is</WSD> possible only because the basic form of such an understanding , for all its vagueness and tendency to mistake the detail , is initially given in the way man feels the world . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)