Mechanism is that which obeys impulses from outside.
Even in his motives and his impulses, in his mental attitude towards outward surroundings, in his appetites and aversions, inherited tendency and environment have been found to play a very large part; indeed many thinkers hold that the whole of a man's development, mental as well.
I regard my pupil as a free and active being, whose own spontaneous impulses must be my surest guide.
Rhyn does understand, but he can't control what he is. Even if he wanted to do good, he'd fail. He's too weak to control his impulses, Kris.
There is a similar difficulty in tracing the paths by which the impulses are transmitted to the growing and curving regions.