From the additional notes to Charles Spurgeon's The Treasury of David, Psalm 119, verse 18
Open thou mine eyes. The saints do not complain of the obscurity of the law, but of their own blindness. The Psalmist doth not say, Lord make a plainer law, but, Lord open mine eyes: blind men might as well complain of God, that he doth not make a sun whereby they might see. The word is "a light that shineth in a dark place" (2 Peter 1:19). There is no want of light in the Scripture, but there is a veil of darkness upon our hearts; so that if in this clear light we cannot see, the defect is not in the word, but in ourselves.
The light which they beg is not anything besides the word. When God is said to enlighten us, it is not that we should expect new revelations, but that we may see the wonders in his word, or get a clear sight of what is already revealed. Those that vent their own dreams under the name of the Spirit, and divine light, they do not give you mysteria, but monstra, portentous opinions; they do not show you the wondrous things of God's law, but the prodigies of their own brain; unhappy abortives, that die as soon as they come to light. "To the law and to the testimony: if they speak not according to this word, it is because there is no light in them" (Isaiah 8:20). The light which we have is not without the word, but by the word.
The Hebrew phrase signifieth "unveil mine eyes." There is a double work, negative and positive. There is a taking away of the veil, and an infusion of light. Paul's cure of his natural blindness is a fit emblem of our cure of spiritual blindness: "Immediately there fell from his eyes as it had been scales: and he received sight forthwith" (Acts 9:18). First, the scales fall from our eyes, and then we receive sight. Thomas Manton.