Heb 4:12
12For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of
soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.
IMO "soul" in this verse speaks of the natural man as here:
1 Cor. 15:42-49
42So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption: 43It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power: 44It is sown
a natural body; it is raised a spiritual body. There is a
natural body, and there is a spiritual body. 45And so it is written,
The first man Adam was made a living soul; the last Adam was made a quickening spirit.
Many times the soul simply refers to man himself which includes a body. Man also has a spirit. The word of God discerns where one starts and the other ends.
The same verse gives a parellism speaking of the joints and marrow (natural man, flesh) and thoughts and intents of the heart, (spirit", inner man).