To the Tungusic peoples of Siberia, a shaman is someone who is born with a peculiar sickness. This isn't a sickness like the flu or cancer, but rather an existential one. A "madness" the haunts the dreams of a person until they come to the full awareness of who they are.
Mircea Eliade, who wrote what is considered to be a seminal work on Shamanic practice put it this way:
The future shaman sometimes takes the risk of being mistaken for a "madman"... but his "madness" fulfills a mystic function; it reveals certain aspects of reality to him that are inaccessible to other mortals, and it is only after having experienced and entered into these hidden dimensions of reality that the "madman" becomes a shaman.
Madness has many forms. To the church, Galileo was a madman for suggesting that the earth was not the center of the universe. To many humans 200 years ago, you would be a madman to suggest that woman stood on equal footing with men. Indeed today, you may be a madman to suggest that material gain may not be good.
Yet, it is often the people we treat as mad that have that new view on reality. A new way of looking at things or experiencing the world. Though often, we're dismissive of these people and unsure of how to relate to them or how we should treat them. Despite this, it seems the madness and genius are often (if not always) two sides of the same coin.
Only psychos and shamans create their own reality - Terrence McKenna
Indeed, it may be the shamans we fear the most. Those who are beyond our system of reasoning, beyond the world as we know it. Yet without them we would know little.
The world and universe itself is often mad. The rules of quantum physics, the incredibly bizarre creatures that inhabit our planet, the many forms of human sexuality, and far more. It seems that the world is often more mad then we can imagine.
This is to those who are sick and who are trying to understand their place. Sometimes sickness doesn't come from disease, but from a poor orientation of our thoughts relative to the world. Embrace those thoughts, embrace the world, and let your madness lead the way to the future.