Pistis consists of thoughts and takes place in the mind. We can have this type of faith and never act upon it. Aman requires that we act as though we believe something. For example, when I was a law enforcement officer, my employer furnished me with a bullet proof vest. They told me that it would stop a bullet and I believed them, but I rarely wore it and would never have staked my life on what they had told me. That is pistis. If I stood up in the middle of a gunfight and said "go ahead and shoot me" that would be aman. Because the Hebrew concept of aman requires action, Paul and James were not at odds over
works vs faith.
As I said earlier, I believe that as Jews, they both understood that although justification is by grace through faith; faith engenders the good works Yahveh created us for in Christ Jesus, which He prepared beforehand for us to walk in. Failure to understand the Hebrew concept of faith that is part of our Jewish heritage has resulted in bad doctrine that holds that all we need is mental ascent and that doing good works is at best unnecessary and at worst pernicious.