The Third Eye of Śiva [Triyambaka] is set mid-point on the forehead, equidistant from the two corporal eyes. In its proper mythic interpretation, the Eye carries no eyelid [an irregularity quickly corrected by Bollywood poster-artists]. It stands unprotected, does not blink, is never closed.
When Hui-neng, the defining sixth patriarch of C’han-Zen arrived on the scene, the traditional practice was to ‘cleanse one’s eye’ until one see’s the immaculate purity of Self-Nature. With Hui-neng, the old symbol of ‘K’an-ching‘, to keep an eye on purity, became ‘Chien-hsing‘.
The former symbol was that of ‘hand and eye’, the separated observer, while Chien-hsing, an eye alone, was pure un-bifurcated ‘Seeing’. It becomes a principal name for Satori [‘Enlightenment’].
And it halted the Tradition from sinking, as had happened to so many others, into a passive contemplation.
There are examples aplenty however where ‘hsing‘ and its variants [fo-hsing: Buddha-Nature; fa-hsing: Dharma-Nature, etc.] are reduced to the Buddhist equivalent of the Vedanthic ‘Self’.
The Third Eye has been incorporated into the Puranic Literature in a myriad imaginative ways [Kamadeva, Trimurthy, et al].
And it has had a wide field of play, from early Buddhist [Swayambunath Temple, Kathmandu, Nepal, above] to the Freemason Pyramid’s: ‘Eye of Providence’, atop the isosceles [Grk: literally, ‘Same Leg’] edges of an equidistant base, which thanks to the philosophical orientation of the Founding Fathers [Jefferson, et al], can be found on the Great Seal of the United States and the One Dollar Bill. Quite a journey.