The four functions are based upon the fact that our consciousness says there is something in the unconscious. Sensation is a sort of perception, it knows the thing is there; thinking tells us what it is; feeling says what it is worth to one… and intuition tells us what it might become, its possibilities.
Jung provides his most concise authoritative definition of the four functions as the exhaustive modes by which consciousness orients itself to the unconscious.
, Dream Analysis: Notes of the Seminar Given in 1928-1930, 1984thesis