Among
Men they were supposed (at first) by those that had dealings with them to be Men who had acquired lore and arts by long and secret study...
Yet they were not so. For they came from over the Sea out of the Uttermost West; though this was for long known only to
Círdan, Guardian of
the Third Ring, master of
the Grey Havens, who saw their landings upon the western shores. Emissaries they were from Lords of the West, the
Valar, who still took counsel for the governance of
Middle-earth, and when the shadow of
Sauron began first to stir again took this means of resisting him. For with the consent of
Eru they sent members of their own high order, but clad in bodies of as of Men, real and not feigned, but subject to the fears and pains and weariness of earth, able to hunger and thirst and be slain; though because of their noble
spirits they did not die, and aged only by the cares and labours of many long years. And this the Valar did, desiring to amend the errors of old, especially that they had attempted to guard and seclude the Eldar by their own might and glory fully revealed; whereas now their emissaries were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men and
Elves by open display of power, but coming in shapes weak and humble were bidden to advise and persuade Men and Elves to good, and to seek to unite in love and understanding all those whom Sauron, should he come again, would endeavour to dominate and corrupt.
For it is said indeed that being embodied the
Istari had needs to learn much anew by slow experience, and though they knew whence they came the memory of
the Blessed Realm was to them a vision from afar off, for which (so long as they remained true to their mission) they yearned exceedingly. Thus by enduring of free will the pangs of exile and the deceits of Sauron they might redress the evils of that time.
Unfinished Tales: Part 4.2