Jeremiah 17:9
The context
Jeremiah is pronouncing judgment on Judah for idolatry and for trusting in alliances and in the arm of flesh. Chapter 17 sets a curse on the one who trusts in man and a blessing on the one who trusts the Lord, then turns to the heart. Verse 10 answers the question with God's searching.
What it establishes
It establishes that the human heart is unreliable as a guide and that its depths are not available to the person who has one. The verse is diagnostic, and it is immediately followed by the claim that God searches the heart and repays according to conduct.
What it does not establish
It does not establish that people cannot know their motives at all. The prophet denies that the heart is a trustworthy court of last appeal, not that self-examination is impossible. Nor does he say the heart is wholly evil by nature.
Short note, first pass, October 2026. Corrections welcome.
Cited in these answers
Part of Scripture, Opened — the exegetical layer behind every citation on this site. Scripture quotations are from the ESV.