Under the 1987 Constitution, international law can become part of the sphere of domestic law either by transformation or incorporation. The transformation method requires that an international law be transformed into a domestic law through a constitutional mechanism such as local legislation. The incorporation method applies when, by mere constitutional declaration, international law is deemed to have the force of domestic law. [Emphasis supplied]

We characterized "generally accepted principles of international law" as norms of general or customary international law that are binding on all states. We held further:¹¹⁷

[G]enerally accepted principles of international law, by virtue of the incorporation clause of the Constitution, form part of the laws of the land even if they do not derive from treaty obligations. The classical formulation in international law sees those customary rules accepted as binding result from the combination [of] two elements: the established, widespread, and consistent practice on the part of States; and a psychological element known as the opinion juris sive necessitates (opinion as to law or necessity). Implicit in the latter element is a belief that the practice in question is rendered obligatory by the existence of a rule of law requiring it. [Emphasis in the original]

The most widely accepted statement of sources of international law today is Article 38(1) of the Statute of the International Court of Justice, which provides that the Court shall apply "international custom, as evidence of a general practice accepted as law."¹¹⁸ The material sources of custom include State practice, State legislation, international and national judicial decisions, recitals in treaties and other international instruments, a pattern of treaties in the same form, the practice of international organs, and resolutions relating to legal questions in the UN General Assembly.¹¹⁹ Sometimes referred to as "evidence" of international law,¹²⁰ these sources identify the substance and content of the obligations of States and are indicative of the "State practice" and "opinio juris" requirements of international law.¹²¹ We note the following in these respects:

G.R. No. 182498               December 3, 2009
GEN. AVELINO I. RAZON, JR., Chief, Philippine National Police (PNP); Police Chief Superintendent RAUL CASTAÑEDA, Chief, Criminal Investigation and Detection Group (CIDG); Police Senior Superintendent LEONARDO A. ESPINA, Chief, Police Anti-Crime and Emergency Response (PACER); and GEN. JOEL R. GOLTIAO, Regional Director of ARMM, PNP, Petitioners,
vs. MARY JEAN B. TAGITIS, herein represented by ATTY. FELIPE P. ARCILLA, JR., Attorney-in-Fact, Respondent.