"To the leader; a psalm of David," says Nabal. Why does the verse mention the heart and kidneys out of all the organs? Because the eyes follow the heart, and the ears, intestines, and 248 organs in a person all follow the heart. The kidneys advise the heart, and the heart decides.
Therefore, only the heart and kidneys are mentioned, and the Lord examines the heart and tests the kidneys. Similarly, it says, "And you, Solomon, my son, know the God of your fathers and serve him with a perfect heart, for all hearts seek the Lord" (1 Chronicles 28:9). What are these "all hearts"? They are the two hearts that a person has, the good inclination and the evil inclination.
Another explanation: "I am the Lord who searches the heart." The wicked say, "We dig deep in our hearts as if the Lord does not know." The Lord says to them, "Woe to those who hide their plans from me" (Isaiah 29:15). You think I am like you, that you can make a likeness to the Creator and plant it for the planter?
"Will you be accounted as clay?" (Isaiah 29:16). Rabbi Levi said, "What is this like?" It is like an architect who built a city and made hiding places and chambers, but in the end, a ruler came to take the listers, and they fled and hid themselves in those rooms. He said to them, "Fools, before whom are you hiding?
I am the artist who built the city, and I know all of its hiding places better than you." So too, the Lord says to the wicked, "Fools, before whom are you hiding? I created man, and I know all the chambers within him. Woe to those who dig deep to hide their plans from the Lord" (Isaiah 29:15).
Rabbi Jeremiah said, "What is meant by 'The heart is deceitful above all things, and it is exceedingly perverse: who can know it?' (Jeremiah 17:9)?" And it is written afterward, "I the Lord search the heart" (Jeremiah 17:10). I search for the hearts and reveal the hidden things. And so Daniel says, "He reveals deep and secret things" (Daniel 2:22).