Spirituality is the key to events throughout The M.D. Early on, Billy learns that he can see aspects of life that other people cannot; if he concentrates and looks just right, he can see a little of the spiritual world that is always active and infusing itself into human experience. This makes him special, and for much of the novel it seems that this is why Mercury is drawn to him. But the spiritual realm is a complex place, and it turns out that Billy did not influence Mercury, that instead Mercury created Billy through generations of genetic manipulation. The point, here, is that the spiritual world can work on people in subtle, profound ways and it is mostly human vanity that makes William and others think that they are actually in control of their spiritual lives. The complex way in which spirituality is worked out in the novel enriches it as a reading experience; no one has a simple spiritual life: For instance, William's complex family extends beyond the living to include the dead; Henry's spirit attempts to identify the source of his family's ills and to help them solve those ills.