Why does criminal law excuse or justify at all — why not punish every harm-causer? The rationales (the essay's opening structure):
1. The capacity principle (culpability): punishment presupposes a responsible agent — free choice, knowledge, rationality (Jurisprudence-II's conditions of liability). The child, the madman, the coerced lack the capacity the criminal censure presupposes; punishing them is not justice but cruelty without purpose;
2. The normative vindication of right: some acts that cause harm are right — the woman who repels her rapist, the person who breaks a lock to escape a fire: punishing them would invert the law's message (law cannot condemn law-abiding resistance to wrongdoing);
3. Deterrence's limits: no threat deters the incapable or the utterly coerced; punishing them deters no one;
4. Humanity and the social contract: a legal system that ignores extremity loses its moral authority — defences are where criminal law acknowledges the human condition;
5. Rule-of-law discipline: defences are codified, bounded, provable — the legality principle's application to exculpation (no "unwritten mercy" but law).