How about that sword that separates and divides asunder soul from spirit?
I usually avoid these discussions, because they are so often unfruitful, but your questions seems sincere and I will try to provide a short sincere answer.
The following verses are examples of how the word "soul" is generally used in the Bible:
Gen. 46:15 These be the sons of Leah, which she bare unto Jacob in Padanaram, with his daughter Dinah: all the souls of his sons and his daughters were thirty and three.
Exodus 1:5 And all the souls that came out of the loins of Jacob were seventy souls: for Joseph was in Egypt already.
Acts 2:41 Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.
Acts 7:14 Then sent Joseph, and called his father Jacob to him, and all his kindred, threescore and fifteen souls.
Acts 27:37 And we were in all in the ship two hundred threescore and sixteen souls.
In general, the word "soul" means the same as "person" or "one's self" and the world "self" (himself, herself, yourself) and "soul" can often be exchanged, as they are in the following:
If you compare the following passages, you will see that Luke uses the word "himself" to refer to the very same thing Matthew amd Mark use the world "soul" to refer to:
Mat. 16:26 For what is a man profited, if he shall gain the whole world, and lose his own soul? or what shall a man give in exchange for his soul?
Mark 8:36 For what shall it profit a man, if he shall gain the whole world, and lose his own soul?
Luke 9:25 For what is a man advantaged, if he gain the whole world, and lose himself, or be cast away?
Now, in the passage you referred to:
Heb. 4:12 For 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.
If we understand the "soul" to be a living person, and the spirit is what gives the flesh life, dividing soul and spirit would mean separating life from the flesh, which would mean the person would die, or cease to be a soul. The soul cannot be separated from a person because it is the person.
This may be too long, and you need not agree with it, but it is clear to me that is what the Bible teaches, although most theologians seem very confused on this (and most other) issues.
Hank