Hi smallstar.
A purpose can be one thing or many things. The posts here offer varied ideas worth considering.
For some people, such as Australian Aboriginals and different Native Indians, purpose is indeed grounded in life itself. Their existence gains meaning though sharing and maintaining cultural practices and traditions. You could also make connections to penguins, birds and animals, whose primary purpose is linked to finding a mate, procreating and surviving.
You mention Napoleon Hill who relates purpose to "money conscious" and "poverty conscious" people. He believes you can realize your purpose 'step-by-step' through will-power and persistence. His ideas of achievement evolve based on how we receive material compensation as well as "the knowledge that every failure brings with it the seed of an equivalent advantage."
