Symbolism, Imagery, Allegory
When Black tells White that he once confessed the worst thing he ever did to a spiritual friend of his, he explains:
"The soul might be silent but the servant of the soul has always got a voice and it has got one for a reason. The life of the master depends on the servant and this is one master that has got to be sustained. Got to be sustained." (104)
What is the "servant of the soul" in this case? We can assume that it's the mind—the part of the self that is responsible for working for the soul (our "master), providing it with the moral and spiritual nutrition it needs to get by—good thoughts, good words, good deeds. But of course, if the mind goes wrong, it can easily starve the soul or stunt its growth. The soul may be the ultimate power, but the mind sustains it, and as such, can do so for good or for ill.