Customary international law consists of unwritten rules that develop from consistent State practice carried out with a sense of legal obligation (opinio juris).

These norms bind States even without a treaty and often protect basic human rights and liberty, especially where written law is silent. In Philippine law, customary international law may be applied directly by courts when consistent with the Constitution and fundamental rights.

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High-yield in Public International Law and Political Law. Frequently tested in problems involving aliens, deportation, detention, human rights, and limits on executive power.

A common trap is assuming aliens have no constitutional or human-rights protection—they do.

Mejoff v. Director of Prisons, G.R. No. L-4254, September 26, 1951 (Supreme Court; Lawphil).

Mejoff, a stateless alien ordered deported, remained detained for years because no country would accept him.

The Supreme Court granted habeas corpus and ordered his release, ruling that while the State may detain an alien pending deportation, detention cannot be indefinite when deportation is no longer reasonably possible. Prolonged detention without a realistic prospect of removal violates individual liberty and fundamental principles recognized under international law.

🥜 The State cannot detain an alien indefinitely for deportation—once removal is no longer feasible, continued detention is illegal.