'Natural' in the Bible

But the natural [unbelieving] man does not accept the things [the teachings and revelations] of the Spirit of God, for they are foolishness [absurd and illogical] to him; and he is incapable of understanding them, because they are spiritually discerned and appreciated, [and he is unqualified to judge spiritual matters].

and all [of them] drank the same spiritual drink, for they were drinking from a spiritual rock which followed them; and the rock was Christ.

it is sown a natural body [mortal, suited to earth], it is raised a spiritual body [immortal, suited to heaven]. As surely as there is a physical body, there is also a spiritual body.