"Trust in the Lord with all thine heart; and lean not unto thine own understanding." (Proverbs 3:5)
Would you say the effectiveness of our walking in the former part of this statement (trusting in the Lord with all our heart) has to do with how closely we follow the latter? Rarely, if ever, do you find the fifth verse of Proverbs chapter three without the next one. Greeting cards, journals, bookmarks, stationery of all types quote the two verses with abandon. And rightfully so because the two together are a dynamite prescription for living out one's life before God. But it's verse five I'd like to emphasize.
"Teach me Thy way, O Lord; I will walk in Thy truth: unite my heart to fear Thy name. I will praise Thee, O Lord my God, with all my heart: and I will glorify Thy name for evermore." (Psalm 86:11) It's interesting how you can switch two letters in the word "unite" and get "untie". It's so easy to get tangled up and tied to things that we have no way of knowing or remembering how we got there. Then again, being "tangled up in God" may not be such a bad idea.
Singleness of heart is essential. Everytime God chided and disciplined and punished His children throughout the Old Testament, it most likely had something to do with getting distracted by the pantheistic panoply affecting the spiritual atmosphere of wherever the children of Israel happened to find themselves. They'd get their hearts tangled and tied up with other (false) spiritual promises and rather than "trust[ing] in the Lord with all [their] heart", they got distracted. We may not heed the call of Baal nowadays. But as Baal promised crops and sustenance in abundance, anything akin to that for our life is essentially the same thing. Are we going to trust God with all our heart? Or do we still rely on something other than God to meet our needs?
That word "lean" (Hebrew: sha'an) means "to support one's self" (Strong's). To "rest yourselves" (Genesis 18:4). Is it too loose a definition to say (in a negative sense) to "become complacent"? Because if we do that very thing--"lean...unto our own understanding"--we will become complacent. America has long been blessed with an abundance of goods and services and provender. And this isn't a call for a knee-jerk asceticism or an irrational rejection of God's goodness and bounty. What it should serve as, if our eyes are open, is inspiration to thank God for what He allows us to enjoy--and then give from that standpoint, should we feel so inclined. This attitude is what kills complacency. And turn it around. While the word translated "trust"--a fuller and more wide-reaching one--is different than the one for "lean", that same thing (leaning) should apply to God. Leaning on Him. Trusting in Him. The simplest explanation of "Trust[ing] in the Lord with all [our] heart", I remember my dad taught me when I was a kid, is that of a chair. You're at rest and reassured. So much so that you don't even think about the fact that the four legs of the chair are supporting your entire weight.
"And should not I spare Nineveh, that great city, wherein are more than sixscore thousand persons that cannot discern between their right hand and their left hand; and also much cattle?" (Jonah 4:11, emphasis mine)
The Hebrew word translated "understanding" is "biynah". From the word "biyn". This is where it gets simple. I'm not talking about allaying the complexity of our life. I'm talking about binary. I'm talking about one, or the other. What Solomon is essentially saying is, "don't rely on your own ability to distinguish between what's truth and what's a lie". Seen in this light, I think the prevailing airy veneer given to this verse and its brother don't do the seriousness of the content justice. It's so, so easy to accept any number of thoughts about life and situations, about God and others, that are absolutely wrong. This is why Solomon says to not "lean" on our own "understanding". Or "rely" on our own "insight" as the New King James words it. Because while we may mean well, only God has the right read on a situation or a person.
And if you find yourself tied up in things and you see no way out. I suppose I can bring in the other verse (Proverbs 3:6): "In all Thy ways acknowledge Him, and He shall direct thy paths."
God can lead you out of any tangle you find yourself in.