The Eye: what was it?

[Inziladun]

Originally Posted by skip spence

A curious thing though is that when Frodo and Sam get a glimpse of the Dark Tower they should be looking straight at the Window of the Eye, since they are close to the entrance it is facing, but The Eye is then turned North, towards Morannon, and since Mount Doom is North-West of Barad-Dûr, they can't see the window directly, it is behind the Tower from their direction. Apparently the Eye is gazing out of not the Window of the Eye, but out of another less renowned window.

There's an interesting parallel with Sauron looking out from Barad-dûr with the Eye.

Then [Sauron] made [Minas Tirith] into a watchtower for Morgoth, a stronghold a evil, and a menace; and the fair isle of Tol Sirion became accursed, and it was called Tol-in-Gaurhoth, the Isle of Werewolves. No living creature could pass through that vale that Sauron did not espy from the tower where he sat.

Silm Of the Ruin of Beleriand

The power of far-seeing seems to be the same, though it is not used with the Eye imagery.
I'm mostly convinced that the Eye was mainly symbolic, a contrivance of Sauron in Third Age 'tyrant' guise to appear more threatening, to his forces and his enemies alike. As someone else may have mentioned, the idea of his Great Eye constantly observing them was probably a considerable motivation for his troops as well.