So, I was in my car the other day at lunch, and I had a thought. I asked God, I said, "God, what is the difference between the body, the mind, and the spirit. I want to know my spirit. I also want to know how the three operate together."
Then this morning I was reading 1 Corinthians 2, specifically verse 11. "For who among men knows the thoughts of a man except the man's spirit within him? In the same way no one knows the thoughts of God except the Spirit of God." NIV
So there is a hint of what the spirit is. It is that part of you that knows your thoughts. I suggest that it is the third part of you that observes you and keeps track of what you think, and how you are going to act on those thoughts. Interesting indeed.