I have often found it fascinating to read vivid and plain descriptions of ordinary things. John Calvin does this very well in most things, especially regarding the faculties God has given man.
Thus let us, therefore hold – as indeed is suitable to our present purpose – that the human soul consists of two faculties, understanding and will. Let the office, moreover, of understanding be to distinguish between objects, as each seems worthy of approval or disapproval; while that of the will, to choose and follow what the understanding pronounces good, but to reject and flee what it disapproves. (Institutes, 1.15.7)
How does the mind play into these faculties?
God provided man’s soul with a mind, by which to distinguish good from evil, right from wrong, and with the light of reason as a guide, to distinguish what should be followed from what should be avoided. (Institutes 1.15.8)
So you see, according to Calvin, the way the God-given faculties work together. First, our understanding distinguishes between the goodness or badness of an object. Second, those things the understanding finds worthy of approval the will chooses to embrace while those things the understanding finds worthy of disapproval the will chooses to turn away from. Here in Calvin, the will and the mind almost play identical roles. I think the difference is that the mind, or the thinking part of man, is what houses both the understanding and the will. How then is this compatible with the soul? The mind is not merely the brain, but the whole faculty of the awareness present in man. Thus, the soul of man (which some could rightly call the “heart”) includes the mind, the understanding, and the will.
This is all brought back to prominent focus when we linger over the thought of what God does inside a person in the salvation of that person. He changes the nature of our soul so that all within us (heart, mind, understanding, will) no longer sees Jesus Christ as foolishness but for the first time sees and savors Jesus Christ as worthy of infinite worship. He makes all of us new. Praise God for His work in man!