Rather than denoting a person who is lacking in mental ability, the word “fool,” as used in the Bible, generally refers to an individual who spurns reason and follows a morally insensible course out of harmony with God’s righteous standards. Various Hebrew terms denoting such a one are kesilʹ (‘stupid one’; Pr 1:22), ʼewilʹ (“foolish one”; Pr 12:15), na·valʹ (‘senseless one’; Pr 17:7), and lets (“ridiculer”; Pr 13:1). The Greek aʹphron refers to an “unreasonable one” (Lu 12:20), a·noʹe·tos to one “senseless” (Ga 3:1), and mo·rosʹ to a ‘fool’ or “foolish” one (Mt 23:17; 25:2).

The course of the man Nabal illustrates the way of a fool (1Sa 25) as does that of people who know the true God and then worship created things. (Ro 1:20-25) Isaiah said a fool, or senseless person, will speak “mere senselessness, and his very heart will work at what is hurtful, to work at apostasy and to speak against Jehovah what is wayward, to cause the soul of the hungry one to go empty, and he causes even the thirsty one to go without drink itself.” (Isa 32:6) The fool despises wisdom and discipline. (Pr 1:7) Instead of heeding counsel, the fool continues walking in a way he considers “right in his own eyes.” (Pr 12:15) He is quick to take offense and bursts out in disputing. (Ec 7:9; Pr 20:3) He says in his heart (his actions indicating what his lips may not say in so many words): “There is no Jehovah.”​—Ps 14:1.

Jesus Christ rightly referred to the scribes and Pharisees as “fools and blind ones,” that is, persons lacking wisdom and being morally worthless, for they had distorted the truth by man-made traditions and followed a hypocritical course. Moreover, Jesus backed up the correctness of this designation by illustrating their lack of discernment. (Mt 23:15-22; 15:3) However, the individual wrongly calling a brother a “despicable fool,” judging and condemning his brother as being morally worthless, would make himself liable to Gehenna.​—Mt 5:22; Ro 14:10-12; Mt 7:1, 2.

The foolish man who built his house upon the sand and the rich man whose land was producing well, and who therefore planned to expand his storage facilities and then really enjoy life, are examples of Jesus’ fine illustrations from daily life highlighting the foolishness of neglecting spiritual things and thereby missing out on the real blessing. Moreover, failing to “keep on the watch” in a spiritual way is folly, as is emphasized by Jesus’ illustration of the five foolish virgins who, in going out to meet the bridegroom, took no oil with them for their lamps.​—Mt 7:24-27; Lu 12:16-21; Mt 25:1-13.

To become truly wise, a person must become a fool in the eyes of the world, “for the wisdom of this world is foolishness with God.” It is not the worldly wise but those looked down upon as persons without knowledge, fools, whom Jehovah has chosen to represent him. This has resulted in making the foolishness of this world even more apparent. Furthermore, this removes all reason for boasting on the part of the favored individual. Instead, all glory goes rightfully to the Source of wisdom, Jehovah.​—1Co 3:18, 19; 1:18-31.

Answering a fool in harmony with or “according to his foolishness” in the sense of resorting to his degrading methods of argument puts the one so doing in agreement with the fool’s unsound reasonings or ways. In order not to become like the fool in this respect, we are counseled by the proverb: “Do not answer anyone stupid according to his foolishness.” On the other hand, Proverbs 26:4, 5 shows that answering him “according to his foolishness” in the sense of analyzing his contentions, exposing them as being ridiculous, and showing that his own arguments lead to entirely different conclusions from those he has drawn can be beneficial.