What is the theme in Dirk Gently's Holistic Detective Agency by Douglas Adams?

Belief and reason play an important role in Dirk Gently's Holistic Detective Agency. The guilty accident that causes the ghost's dilemma and sets in motion the development of life on Earth is founded in the fundamental problem of facing what's true versus believing what you want to believe. The ghost wants to believe that the engines are fixed, and so he sends an Electric Monk to tell him what he wants to believe. The Electric Monk is essentially a yes-man, a self-affirmation instead of a check of objective facts. The result is a disaster.