A priest is a functionary of a social sort. The society worships certain deities in a certain way, and the priest becomes ordained as a functionary to carry out that ritual. The deity to whom he is devoted is a deity that was there before he came along. But the shaman's powers are symbolized in his own familiars, deities of his own personal experience. His authority comes out of a psychological experience, not a social ordination.
Joseph Campbell, The Power of Myth
insightful