Human existence provides each of us certain things that are uniquely our own. That genetic code which directed the formation of this existence is the only one walking around, that is unless your a twin, etc. Fingerprints are another item that belongs to this set of things that are one of a kind. Psychology would teach us that personality is "...the unique organization of enduring attributes of the individual."
Now Webster defines this as the organization of the individual's distinguishing character traits, attitudes, or habits. As early as 400 B.C., Hippocrates thought that certain "humors" [blood, black bile, yellow bile, and phlegm] produced a certain temperament within the individual. Now a days, we have certainly expanded our concept of this thing called personality, but each of us has one...another common thread.