EYE
It is used symbolically of God’s omnipresence: “The eyes of the Lord are in every place” (Proverbs 15:3). “The eyes of the Lord are upon the righteous” (Ps. 34:15; 1 Pet. 3:12; cf. 2 Chron. 16:9; Zech. 4:10). His eyes are on the wicked, and his eye will not forgive nor have compassion on the day of judgment (Ez. 5:11).
The eye is also used symbolically of the organ that transmits light to the soul. If the eye is single (meaning there is only one object, the glory of God, before the soul), the whole body is full of light; But if the eye is bad, having a scattering of objects (as when the eye sees double), the whole body will be full of darkness.
And if the light (even if it is true light) becomes darkness, this darkness is truly black (cf. Mt. 6:22, 23). A Christian in this state may be doing those very things that he condemns in others.
The Christian is exhorted not to serve the eye (Eph. 6:6), which means that he must act with diligence and good spirit in fulfilling obligations, and not because he fears vigilance.