To understand the concept of Filipino citizenship, especially in the context of Political Law and Public International Law, we must delve into the provisions of the 1987 Philippine Constitution, applicable statutes, and relevant jurisprudence. Here’s a detailed breakdown:

I. Definition of Citizenship

Citizenship refers to the legal status of being a member of a particular state, in this case, the Philippines, and it entails specific rights and obligations under that state’s law. Nationality and citizenship are often used interchangeably, though nationality is a broader concept related to the individual's membership in a nation, while citizenship focuses on the legal relationship with the state.

II. Primary Sources of Law on Citizenship

The 1987 Philippine Constitution (Article IV, Citizenship)
Legislation:
- Commonwealth Act No. 63
- Republic Act No. 9225 (Citizenship Retention and Re-acquisition Act of 2003)
- Presidential Decree No. 725
Relevant Jurisprudence from the Supreme Court on citizenship matters.

III. Citizenship under the 1987 Constitution

A. Who Are Filipinos (Article IV, Section 1 of the 1987 Constitution)

There are five categories of individuals recognized as Filipino citizens under the Constitution:

Those who are citizens of the Philippines at the time of the adoption of this Constitution.
- Reference: Article IV, Section 1 (1).
- This pertains to individuals who were recognized as Filipino citizens under previous constitutions (1935, 1973) and relevant laws prior to the ratification of the 1987 Constitution.
Those whose fathers or mothers are citizens of the Philippines.
- Reference: Article IV, Section 1 (2).
- This adopts the principle of jus sanguinis (right of blood), meaning citizenship is acquired by birth to Filipino parents. Under Philippine law, it is immaterial whether the Filipino parent is the mother or father.
Those born before January 17, 1973, of Filipino mothers, who elect Philippine citizenship upon reaching the age of majority.
- Reference: Article IV, Section 1 (3).
- The 1935 Constitution recognized jus sanguinis but was patrilineal, meaning citizenship was determined by the father's nationality. The 1973 and 1987 Constitutions allowed children born to Filipino mothers before January 17, 1973, to elect Philippine citizenship upon reaching the age of 18, thus curing the historical bias.
Those who are naturalized in accordance with law.
- Reference: Article IV, Section 1 (4).
- Naturalization is a legal process whereby a foreign national acquires Filipino citizenship. It can be done via judicial or administrative procedures, and more recently, through special laws like Republic Act No. 9225.
Those who are foundlings.
- Jurisprudence: The Supreme Court, in cases such as Roa v. Collector of Customs and Poe-Llamanzares v. COMELEC, recognized the principle of presumed Filipino citizenship for foundlings, based on principles of humanitarian law and jus soli (right of soil).

IV. Principles of Citizenship in the Philippines

A. Jus Sanguinis (Right of Blood)

The primary principle governing Philippine citizenship is jus sanguinis, under which a person’s citizenship is determined by their parentage, not the place of birth. Anyone born to at least one Filipino parent, regardless of where they are born, is a natural-born Filipino.

B. Natural-born Citizenship

Natural-born citizens are those who are citizens of the Philippines from birth, without needing to perform any act to acquire or perfect their citizenship.
A critical aspect of being natural-born is relevant for public office, as the Constitution mandates that certain officials (e.g., the President, Vice President, Senators, and Members of the House of Representatives) must be natural-born citizens.

V. Modes of Acquiring Citizenship

There are two primary modes of acquiring Filipino citizenship:

A. By Birth

Natural-born citizenship through Filipino parents (jus sanguinis).
By Election: For individuals born to Filipino mothers before January 17, 1973, they may elect Filipino citizenship within a reasonable time after reaching the age of majority (18 years old). Election is a form of exercising the right to choose Filipino citizenship.

B. By Naturalization

Naturalization is a legal process where a foreign national becomes a Filipino citizen. There are three modes of naturalization in the Philippines:

Judicial Naturalization under Commonwealth Act No. 473 (Revised Naturalization Law):
- A judicial proceeding where a foreign national files a petition in court, subject to requirements and residency stipulations.
Administrative Naturalization under Republic Act No. 9139 (Administrative Naturalization Law of 2000):
- For those who have completed specific residency and employment requirements, and who are residents of the Philippines since birth.
Legislative Naturalization:
- Granted by special laws enacted by Congress. These laws confer Filipino citizenship on specific individuals, often in recognition of their contributions to the country.

VI. Modes of Losing and Re-acquiring Filipino Citizenship

A. Loss of Filipino Citizenship

Filipino citizens may lose their citizenship in several ways:

By Naturalization in a Foreign Country:
- Under the 1935 Constitution and prior, Filipinos who were naturalized in other countries automatically lost their Filipino citizenship.
By Express Renunciation of Citizenship:
- Filipinos may renounce their citizenship before foreign authorities for specific purposes (e.g., employment in foreign governments) or to meet the requirements of dual citizenship in other countries.
By Subscribing to an Oath of Allegiance to a Foreign Government:
- Under Commonwealth Act No. 63, subscribing to an oath of allegiance to another country results in the automatic loss of Filipino citizenship.
By Cancellation of Naturalization:
- If a Filipino’s naturalization is later found to have been obtained fraudulently, it may be cancelled by the courts.

B. Re-acquisition of Filipino Citizenship

Republic Act No. 9225 (The Citizenship Retention and Re-acquisition Act of 2003) allows Filipinos who have lost their citizenship through foreign naturalization to re-acquire it. Notable features include:

Dual Citizenship: Under RA 9225, Filipinos who acquire foreign citizenship are allowed to retain their Philippine citizenship by taking an oath of allegiance to the Republic of the Philippines. This effectively results in dual citizenship.
Rights and Obligations of Dual Citizens:
- Dual citizens enjoy full civil and political rights, including the right to vote and the right to hold public office (subject to specific requirements and disqualifications).

VII. Foundlings and Citizenship

A special case in Philippine citizenship involves foundlings—children of unknown parentage. The 1987 Constitution does not specifically mention foundlings, but jurisprudence has established that:

Presumption of Natural-born Status: Foundlings found in Philippine territory are presumed to be natural-born Filipinos unless proven otherwise. The presumption rests on international conventions and customary law.
The Poe-Llamanzares case (2016) was significant in affirming the right of foundlings to be considered natural-born Filipinos, thus eligible for certain positions in government.

VIII. Relevant Jurisprudence on Citizenship

Tecson v. COMELEC (2004): Clarified the citizenship of Fernando Poe Jr., recognizing him as a natural-born Filipino despite issues on the legitimacy of birth.
Valles v. COMELEC (2003): On election of citizenship, the Court clarified that the election must be made upon reaching the age of majority and within a reasonable time thereafter.
Poe-Llamanzares v. COMELEC (2016): Affirmed the natural-born status of foundlings under international and domestic law.

Conclusion

Citizenship in the Philippines is a complex subject governed by constitutional provisions, legislative enactments, and significant jurisprudence. The principle of jus sanguinis is the backbone of Filipino citizenship, and legal processes like naturalization and re-acquisition (through dual citizenship) reflect the flexibility and evolving nature of the law. Various Supreme Court rulings have further clarified the nuances of these laws, especially in cases involving political rights and candidacy for public office.