The Greek adjective aphron means foolish, senseless, or mindless — one who lacks understanding or wisdom. It appears eleven times in the New Testament. Jesus uses it to rebuke the Pharisees (Luke 11:40) and in the parable of the Rich Fool (Luke 12:20: "You fool! This very night your life will be demanded from you"). Paul uses it of himself ironically in 2 Corinthians and of believers before conversion in Titus 3:3.
Biblical foolishness is not primarily intellectual deficiency — it is the practical atheism of living as if God does not matter. The aphron rich man (Luke 12:20) was financially successful and practically shrewd, yet God called him a fool because he had planned for every future except the one that mattered: his account before God. Psalm 14:1 says "The fool says in his heart, 'There is no God.'" The fool in Scripture is not stupid but spiritually blind — refusing to orient life around eternal realities.