Charles H. Spurgeon on Psalms 7:9

"The righteous God trieth the hearts and reins." As common experience shows that the workings of the mind, particularly the passions of joy, grief, and fear, have a very remarkable effect on the reins or kidneys. (See Proverbs 23:16; Psalm 73:21), so from their retired situation in the body, and their being hid in fat, they are often used to denote the most secret workings and affections of the soul. And to "see or examine the reins," is to see or examine those most secret thoughts or desires of the soul. John Parkhurst, 1762.

Verse 9 (last clause). "The righteous God trieth the hearts and reins."

"I that alone am infinite, can try

How deep within itself thine heart doth lie.

Thy seamen's plummet can but reach the ground,

I find that which thine heart itself ne'er found. Francis Quarles, 1592-1644.