” … In the case of each triad, a fourth figure is often added, Ninlil, originally the consort of Enlil, or Nin-makh (“the great lady”) to the first, and Belit (“the lady”) or Ishtar to the second, both, however, symbolizing the female element which, fructified by the male, is the indispensable complement to the production of life, vegetation, fertility and all blessings that go with the never ending process of vitality, growth, decay and regeneration in nature.
This leads us to a consideration, before leaving the pantheon, of one notable female figure, the great mother-goddess, frequently identified with the earth viewed as a fruitful mother but who should rather be regarded in a still wider sense as the mother of all that manifests life, embracing therefore the life in man and the animal world as well as in the fields and mountains in nature in general.
This natural association of a female element as a complement to the male one leads to assigning to every deity a consort who, however, has no independent existence. So Enlil has at his side Nin-lil, Ninib has Gula A (“the great one”), Ningirsu has Bau, Shamash has A, Sin has Nin-gul, Nergal has Laz, Anu a female counterpart Antum, to Ea a consort Shala (“the woman”) is given, to Marduk, Sarpanit or Nin-makh (“the great lady”), to Nabu, Tashmit (“obedience”), while Ashur’s consort appears as Nin-lil or Belit and at times as Ishtar.
All these figures with the single exception of Ishtar are merely shadowy reflections of their male masters, playing no part in the cult outside of receiving homage in association with their male partners. Ishtar, however, although assimilated in the Assyrian pantheon as the consort of Ashur, is an independent figure, who has her own temples and her distinct cult. She appears under a variety of names: Nana, Innina, Irnini, Ninni, Nina all of which contain an element having the force of “lady,” as is also the case with Nin-makh and Nin-lil, likewise used as epithets of the great mother-goddess. Corresponding to the Sumerian element, we have in Akkadian Belit, “lady” or “mistress,” as one of the generic designations of Ishtar.
All this confirms the view that Ishtar is merely the symbol of the female element in the production of life, and that the specific name is of secondary significance. The circumstance that Ninlil, the consort of Enlil, is also (though in texts of a later period) identified with the mother-goddess would seem to show that the female associate of the head of the pantheon was always an Ishtar, though in a certain sense, as we have seen, the consorts of all the gods were Ishtars.”
Morris Jastrow, The Civilization of Babylonia and Assyria, 1915, pp. 232-4.