This is probably another proverbial saying and it answers the question asked at the end of Jeremiah 17:9. I the Lord search the heart and test the mind.
No human can unravel the sinful human heart, but the Lord can. He sees into the very core of every human and not only that, but he tests and judges what he sees. The Hebrew word translated as mind
literally means kidneys and is used to refer to the ethical and emotional responses that come from deep inside a person.1 The Lord knows what is in a person’s heart and is therefore able to rightly and perfectly judge a person’s actions because he sees the beliefs, desires and motives behind every action. He can therefore give every man according to his ways, according to the fruit of his deeds.
Taken together Jeremiah 17:9–10 form the heart, so to speak, of Jeremiah 17:1–13. The contrast of these two verses explains the contrast of the whole passage: sinful, deceitful, faithless humanity in contrast to a holy, just and faithful God.2
10 Moi, l'Eternel, j'éprouve le cœur, je sonde les reins, Pour rendre à chacun selon ses voies, Selon le fruit de ses œuvres.