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 is possible only because the basic form of such an understanding , for all its vagueness and tendency to mistake the detail , is <WSD>initially</WSD> given in the way man feels the world . 
Meaning of the word marked with <WSD> tags:  at the beginning