“The heart is deceitful above all things, and desperately sick; who can understand it?”
Jeremiah doesn't say the heart sometimes deceives — he says it's deceitful above all other things, which means it outpaces even the outside lies we fear.
'Desperately sick' describes a condition that doesn't get fixed by more willpower — it's a sickness requiring a physician from outside.
'Who can understand it?' isn't empty rhetoric — it's the admission that not even the owner of the heart can self-examine with full accuracy.
If the heart deceives itself, the only reliable way out is someone who sees it from the outside — which is exactly what the next verse, about God who searches the heart, offers.
Recognizing that your own heart can deceive you is, paradoxically, the first step of wisdom — only someone who knows they can be fooled seeks counsel from outside.
Act: Pray asking God to reveal one specific area today where your heart may be deceiving you, and seek the honest input of someone you trust about it.