Note 6 at Eph. 4:18: The English word "understanding" was translated from the Greek word DIANOIA meaning "deep thought." This word is specifying more than just thought. It is referring to comprehension, discernment, and judgment. People can function without understanding, but not very well.

Psalm 32:9 says, "Be ye not as the horse, or as the mule, which have no understanding: whose mouth must be held in with bit and bridle, lest they come near unto thee." A horse thinks, but it doesn't seem to have any understanding. People who have their understanding darkened live like animals.

There are 53 times the word "understanding" is mentioned in the book of Proverbs. Wisdom is the principal thing and understanding is next unto it (Prov. 4:7). Walking in the vanity of our minds (see note 5 at v. 17, p. 1121) leads to clouding the understanding with the end result being the hardening (see note 10 at Mk. 6:52, p. 238; see note 3 at Mk. 8:17, p. 258) or blinding (see note 9 at this verse) of our hearts.

Note 7 at Eph. 4:18: Paul spoke about our understanding in 1 Corinthians 13:11. Then in the next verse, he said we all "see through a glass, darkly," describing man at his best as being very limited in understanding. If we are already restricted in our understanding, we sure don't need anything to darken it further. We should avoid walking in the vanity of our minds at all costs (see note 5 at v. 17, p. 1121).

Note 8 at Eph. 4:18: Notice that this alienation wasn't God's choice. It is the ignorance within people that alienates them from the life of God. The Lord "so loved the world, that he gave" (Jn. 3:16). His grace has appeared to all men (Ti. 2:11), but not all men receive it because of the ignorance within them.

Note 9 at Eph. 4:18: This blindness of the heart that Paul is speaking of here is the same thing that the gospel writers spoke of as "hardness of heart." W.E. Vine defines the Greek word POROSIS, which was translated "blindness" here, as "a covering with a callus, a hardening." John 12:40 describes spiritual blindness and hardness of heart as adversely affecting our understanding.