“sense.” The Hebrew word is leb (#03820 לֵב), which is often translated “heart,” but this is one of those cases where that translation would cause confusion. In modern English, the word “heart” usually refers to emotion or passion, but that is not its meaning here. The function of the brain was unknown in biblical times, so things that we generally assign to the brain, like thinking, attitudes, understanding, and good sense, were assigned to the heart.
In this case, the man who commits adultery does not lack emotion or passion, in fact he probably has plenty of that. What he lacks is “good sense.” The range of meaning of the Hebrew word “heart” in this context is reflected in the number and variation in the English translations of this verse: “void of understanding” (ASV; Darby); “without all sense” or “lacks sense” (CJB; HCSB; ESV); “lacks wisdom” (NET); “lacks judgment” (NIV84); “is an utter fool” (NLT); and the Amplified Bible says, “lacks heart and understanding (moral principle and prudence).” There is no way to capture the full meaning of the Hebrew word leb in this verse; there is simply not an English word that carries the same connotations as the Hebrew word “heart.”
Thankfully, lacking good sense is a correctable problem if the person would begin to listen to Wisdom and act on what she says. In fact, that is part of the purpose of Proverbs—to teach people wisdom and good sense. If a person refuses to learn wisdom, then he is guilty before God on Judgment Day, which has very serious consequences, possibly even everlasting death. The seriousness of lacking sense and acting foolishly is an important reason that believers have a responsibility to try to get people to believe in God, get saved, and then begin to acquire good sense. [For more on the Hebrew word leb and “heart,” see commentary on Prov. 15:21, “sense”].