Man's soul is the Lord's Lamp
The soul in his midst
testifies concerning him in judgment [Rashi] Just as a
person searches in the dark with a lamp, so does G--d
search a person's innermost thoughts through his soul,
which knows all and testifies concerning him in judgment.
[Mezudath David] Rabbenu Yonah explains that man is
honored by God with the possession of a holy soul, which
emanates from on high, and therefore he is subject to His
closest scrutiny to the extent that He searchs man's
innermost thoughts'. Hence, in effect, the soul of man is
God's lamp, the enabler of God's scrutiny over him.