Sentence:  What Sam Rayburn 's life <WSD>proves</WSD> to us all is the magnificent lesson in political science that one can devotedly and with absolute dedication represent the seemingly provincial interests of one 's own community , one 's own district , one 's own State , and by that help himself represent even better the sweep and scope of the problems of this the greatest nation of all time . 
Meaning of the word marked with <WSD> tags:  establish the validity of something, as by an example, explanation or experiment