Sentence:  "Look here, sir, this is all very well," he began, "but why can't I fall in love with your daughter? I'm a Doctor of Science and all that sort of thing. I've a perfectly decent outlook. My father was rather a swell in his science. I'm an entirely decent and respectable person." / "I <WSD>beg to differ</WSD>," said Mr. Pope. / "But I am." / "Again," said Mr. Pope, with great patience, and a slight forward bowing of the head, "I <WSD>beg to differ</WSD>." 
Meaning of the word marked with <WSD> tags:  To offer an opposing opinion humbly.