Earlier in the story, the Englishman tells Santiago that one is closest to the Soul of the World when one yearns exclusively for a specific purpose. The Englishman also contends that the Soul of the World is always a positive force.
Santiago discovers that there is only one way to find the Soul of the World, and it is through alchemy. Alchemists believe that a metal can be purified through fire. Once purified, the metal will be free of its individual properties and become the Soul of the World. The Soul of the World is supposedly filled with all wisdom and has knowledge of all earthly things. The alchemists call the discovery of the Soul of the World the Master Work. They believe that turning lead into gold will give them the Soul of the World.
By extension, once humans have the Soul of the World, they can acquire the wisdom to become their best selves.
Later in the story, Santiago meets the sun and has a conversation with it. He explains why alchemy exists and what alchemists do.
Santiago says that alchemy exists to inspire people to become their best selves. He likens the journey of transforming into one's best self to a search for treasure. Once our treasure or purpose is found, we will be inspired to grow mentally and spiritually. We will be inspired to become better than we were before.
Santiago contends that alchemists have a special purpose in life. They labor to turn lead into gold. It is an arduous process, but they are undaunted by the challenges they face. The process of turning lead into gold is a mission, a constant push to improve and innovate. When alchemists strive against all odds to turn lead into gold, they are setting the rest of us an example.
According to Santiago, the alchemists are showing us that, when we strive to become our best selves, we also inspire others to change. In turn, the world becomes a better place.