Why can’t we just do whatever feels good to us? Why can’t we just do whatever makes sense to our own reason? Why can’t we just be guided by tradition, doing things the way we’ve always done them?

Today’s passage gives us two reasons. One we saw yesterday in Psalm 1, to which Jeremiah obviously alludes in verses 5-8. Those who trust in themselves, those who trust in anyone or anything in the fleshly, material world have, by definition, not put their trust in God. They have turned away from God, seeking other sources of truth instead. But since God is the ultimate source of all life, that means they are like a shrub in the desert. No matter how strong or wealthy or successful they think they are, without God they will eventually dry up and die.

But there’s another reason that we shouldn’t exclusively trust our own judgment or feelings. For verse 9 reminds us that our heart is deceitful, and that means it is too easy for us to confuse reason with rationalization. Our heart is sick with sin, naturally inclined not to seek God, but to seek our own good, naturally inclined to pull away from God and from other people. We simply cannot rely on our own thoughts and feelings to lead us in the way we should go.

That’s why we need an external measure of truth, something to test whether our thoughts and feelings do in fact line up with God’s will for us. That’s why we need to trust not in ourselves but in God (v. 7). For God alone is able to reveal the deepest feelings in our hearts. God never confuses rationalization with reason. And in His Word God has given us a never-failing guide for life, a truly infallible rule or measure of all our faith and practice.

Jeremiah 17:5-10 (ESV)

5 Thus says the LORD: “Cursed is the man who trusts in man and makes flesh his strength, whose heart turns away from the LORD.
6 He is like a shrub in the desert, and shall not see any good come. He shall dwell in the parched places of the wilderness, in an uninhabited salt land.
7 “Blessed is the man who trusts in the LORD, whose trust is the LORD.
8 He is like a tree planted by water, that sends out its roots by the stream, and does not fear when heat comes, for its leaves remain green, and is not anxious in the year of drought, for it does not cease to bear fruit.”
9 The heart is deceitful above all things, and desperately sick; who can understand it?
10 “I the LORD search the heart and test the mind, to give every man according to his ways, according to the fruit of his deeds.”