Yagyu Munenori, 1571 - 1646, in The Way of the Living Sword
That part inside each of us that makes plans, determines actions, and commits us to a path is called the mind. That part inside each of us that carries out plans, fulfills actions, and walks the path is called the spirit. The mind is the ultimate master of the body, and the spirit is its servant, to carry out the directions of the mind. The mind uses the spirit to accomplish all that it does. If the spirit acts too much on its own, you will fail. Insure you commit all things to your mind and strive to bring your spirit under subjection of your mind and the two will work as a seamless duo.
Interesting. Logical thinking and willpower do seem to use different parts of your brain, and yet the former can influence and control the latter with practice. Book is highly recommended, thanks to J.D. Rosemont for the recommendation.