For what man intuitively knows the things of a man except the spirit of the man in him? Thus, also the things of God no one knows except the Spirit of God (1 Corinthians 2:11).
The human nature is made up of three parts: body, soul, and spirit (1 Thessalonians 5:23). The soul is our emotional center, whereas the spirit is the basis for our rationality and logic. It is the spirit of a man that has the ability to know the things within a man. This is not experiential knowledge, which requires outward actions; rather, it is the knowledge of the internal thoughts, desires, cravings, and determinations within the heart.
The grace believer is saved in his spirit through the washing and regeneration by the Holy Spirit (Titus 3:5). This new birth happens within the spirit of man and enables him to understand the things of God because he is now connected to God in his rational center (1 Corinthians 6:17; John 4:24).