Exestin (ἔξεστιν) is an impersonal verb meaning "it is lawful/permitted." From ex + eimi (to be), it asks the question of moral and legal permissibility. It appears 31 times in the NT, often in Pharisaic challenges to Jesus and Paul's ethical discussions.
"Is it lawful [exestin]?" is the Pharisees' persistent challenge to Jesus. Is it lawful to heal on the Sabbath (Matt. 12:10)? To pay taxes to Caesar (Matt. 22:17)? For a man to divorce his wife (Matt. 19:3)? Each question tests not just law but Jesus' authority to interpret law. Paul reframes exestin in 1 Corinthians 6:12 and 10:23: "Everything is permissible [exestin] for me — but not everything is beneficial." He shifts from legal permissibility to moral fruitfulness: the question is not just "can I?" but "does it build up?" Liberty is real but love limits its exercise.
Exestin marks the boundary between rule and freedom, law and grace. Jesus consistently refuses its reductive logic: the question "is it lawful?" misses the deeper question of what love requires. Paul in 1 Corinthians uses it in a slogan his readers apparently knew ("everything is permitted") and corrects it twice with "but not everything builds up." Christian ethics is not permission-based but formation-based: not what I can get away with, but who I am becoming.