The priest is the socially initiated, ceremonially inducted member of a recognized religious organization, where he holds a certain rank and functions as the tenant of an office that was held by others before him, while the shaman is one who, as a consequence of a personal psychological crisis, has gained a certain power of his own. The spiritual visitants who came to him in vision had never been seen before by any other; they were his particular familiars and protectors …
The realm of myth, from which, according to primitive belief, the whole spectacle of the world proceeds, and the realm of shamanistic trance are one and the same. Indeed, it is because of the reality of the trance and the profound impression left on the mind of the shaman himself by his experiences that he believes in his craft and its power … This relationship of the shaman’s inner experiences to myth is a supremely important theme and problem of our subject. For if the shaman was the guardian of the mythological lore of mankind during the period of some five or six hundred thousand years when the chief source of sustenance was the hunt, then the inner world of the shaman must be assumed to have played a considerable role in the formation of whatever portion of our spiritual inheritance may have descended from the period of the paleolithic hunt. We must consider, therefore, what the visions within, and springing from, the shamanistic world of experience may have been.
Joseph Campbell, The Masks of God, Vol. I: Primitive Mythology, pp. 231, 250-251