1 Corinthians 2:14 (NKJV)
14 But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.
Who is the natural man? The Greek word is ψυχικός [psuchikos /psoo·khee·kos/]. It is translated four times as natural and two times as sensual. One of the uses of sensual is in James 3:15 where James puts it in line with “earthly, sensual, demomic.” The natural man is one who gives in to his sensous nature and appetite. In Jude, he equates the natural man as one who is “worldly, not having the Spirit.” This then is an unbeliever. The natural man is the the one who thinks of the cross of Christ as foolish.
When anything spiritual is presented to the natural man, he does not receive them. The natural man, when presented with the truth, slams the door in your face. He won’t accept it, he won’t acknowledge the truth of the gospel. An example would be of the natural man falling off a cliff and you extend your hand to save him–he does not take your hand.
He does not receive the things of the Spirit. As mentioned, they are foolishness to him. In fact, neither can he know them. The word know is the Greek word γινώσκω [ginosko /ghin·oce·ko/] meaning he simply cannot grasp the things of the Spirit. He cannot be intimate with spiritual things. Why? Because he needs the aid of the Holy Spirit whom he has received the spirit of the world (2:12) rather than the spirit which is of God. It is only by the Spirit that we can receive the things of the Spirit of God.