In Mijares v. Ranada,¹⁵ the Court held thus:

[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.¹⁶ (Emphasis supplied)

"Generally accepted principles of international law" refers to norms of general or customary international law which are binding on all states,¹⁷ i.e., renunciation of war as an instrument of national policy, the principle of sovereign immunity,¹⁸ a person's right to life, liberty and due process,¹⁹ and pacta sunt servanda,²⁰ among others. The concept of "generally accepted principles of law" has also been depicted in this wise:

Some legal scholars and judges look upon certain "general principles of law" as a primary source of international law because they have the "character of jus rationale" and are "valid through all kinds of human societies." (Judge Tanaka in his dissenting opinion in the 1966 South West Africa Case, 1966 I.C.J. 296). O'Connell holds that certain priniciples are part of international law because they are "basic to legal systems generally" and hence part of the jus gentium. These principles, he believes, are established by a process of reasoning based on the common identity of all legal systems. If there should be doubt or disagreement, one must look to state practice and determine whether the municipal law principle provides a just and acceptable solution. x x x ²¹ (Emphasis supplied)

Fr. Joaquin G. Bernas defines customary international law as follows:

Custom or customary international law means "a general and consistent practice of states followed by them from a sense of legal obligation [opinio juris]." (Restatement) This statement contains the two basic elements of custom: the material factor, that is, how states behave, and the psychological or subjective factor, that is, why they behave the way they do.

x x x x

The initial factor for determining the existence of custom is the actual behavior of states. This includes several elements: duration, consistency, and generality of the practice of states.

The required duration can be either short or long. x x x

x x x x

Duration therefore is not the most important element. More important is the consistency and the generality of the practice. x x x

x x x x

Once the existence of state practice has been established, it becomes necessary to determine why states behave the way they do. Do states behave the way they do because they consider it obligatory to behave thus or do they do it only as a matter of courtesy? Opinio juris, or the belief that a certain form of behavior is obligatory, is what makes practice an international rule. Without it, practice is not law.²² (Underscoring and Emphasis supplied)

Clearly, customary international law is deemed incorporated into our domestic system.²³

G.R. No. 173034             October 9, 2007
PHARMACEUTICAL AND HEALTH CARE ASSOCIATION OF THE PHILIPPINES, petitioner,
vs. HEALTH SECRETARY FRANCISCO T. DUQUE III; HEALTH UNDER SECRETARIES DR. ETHELYN P. NIETO, DR. MARGARITA M. GALON, ATTY. ALEXANDER A. PADILLA, & DR. JADE F. DEL MUNDO; and ASSISTANT SECRETARIES DR. MARIO C. VILLAVERDE, DR. DAVID J. LOZADA, AND DR. NEMESIO T. GAKO, respondents.