Sentence:  "I see in him [Moby Dick] outrageous strength, with an inscrutable malice sinewing it. That inscrutable thing is chiefly what I hate; and be the white whale <WSD>agent</WSD>, or be the white whale principal, I will wreak that hate upon him." 
Meaning of the word marked with <WSD> tags:  One who acts for, or in the place of, another (the principal), by authority from him/her; someone entrusted to do the business of another