Philosophy and reality
The reader must consider what ties together Leverkühn's intermittent fascination with either music or philosophy. Sometimes, he concerns himself only with religion and philosophy, desperately trying to find some sort of language. Then, other times he decides philosophy is not suitable for his interests because language lacks the immediacy of meaning that music contains. He turns to composing works of musical brilliance. In either case, he is seeking to express his point of view, that is, his experience of reality. As a genius, the young man is private and secretive since a young age, and his entire life is a narrative about communicating the unique meaning he enjoys in his mind.
The left-handed path
Adrian is not what might be called a "good" person. He does not abide by the moral strictures of any world view, not because he is evil, but because he is so convinced by his desire to express his point of view that any experience that seems to lend itself to those purposes is deemed ethically strategic in him. When he realizes that he wants to fall in love with a dazzling prostitute, he does not let religious belief (or medical concern) stop him. Throughout his battle with syphilis, he journals about his encounters with Satan, his harassment by demons, and his schizophrenic epiphanies. Notice also that his original professor of religion taught him not about order and tradition, but about chaos magic, the history of alchemy, and the play of the demons in the mind of man.
Mental health and loneliness
Stepping aside from the obvious characteristics about Leverkühn that point toward his nearly-schizophrenic (or perhaps just schizophrenic) genius, there is another aspect of his mind in the novel that carries thematic weight. What is it that makes Adrian so desperate and tortured throughout the book? Why does he care about expressing his unfathomable experience of intelligence and perceptiveness? It is because he is still just a human being with the same needs for community and understanding. Time and time again, he finds company only with those who are fascinated by his abilities or his mind. His desire to be understood is limited by the intellectual limitations of normal communities because he has opinions on what it would mean to be "loved." The only person with whom Adrian reports feeling deep community and love is the femme fatale who gives him the disease that ends up killing him. It is thematic that he falls in love with the deliverer of death.
