Hebrews 4:12 says: "For the word of God is living and powerful, and sharper than any two-edged sword, piercing even to the division of soul and spirit, and of joints and marrow, and is a discerner of the thoughts and intents of the heart."
The expression "soul and spirit" needs to be understood in the context in which it is written. The point of the verse is not that it's possible to separate soul (Greek, psuche) from spirit (Greek, pneuma). The words actually have similar meanings in the Greek. Rather, the point is that God's Word is alive, powerful and sharp.
The New International Commentary on the New Testament volume on Hebrews by F.F. Bruce mentions these phrases "are to be understood as a 'rhetorical accumulation of terms to express the whole mental nature of man on all sides.'" If we fervently and sincerely study God's Word, the Holy Bible, it will help us to see the wrong thoughts and motives within us that must be changed in order to come to have the mind of Jesus Christ.
For more information, please read our booklet Heaven and Hell: What Does the Bible Really Teach?