Jamieson, Fausset & Brown

what man, &c.--literally, "who of men knoweth the things of a man, save the spirit of that man?" things of God knoweth no man--rather, "none knoweth," not angel or man. This proves the impossibility of any knowing the things of God, save by the Spirit of God (who alone knows them, since even in the case of man, so infinitely inferior in mind to God, none of his fellow men, but his own spirit alone knows the things hidden within him).