In the Philippine legal system, the right to self-preservation is a fundamental principle recognized by the Revised Penal Code (RPC). When an individual is faced with a threat to their life, limb, or honor—specifically in crimes against person (such as Homicide or Physical Injuries) and crimes against chastity (such as Attempted or Frustrated Rape)—the law provides a justifying circumstance: Self-Defense.

Under Article 11, Paragraph 1 of the RPC, anyone who acts in defense of their person or rights incurs no criminal liability, provided specific legal requirements are met.

1. The Essential Requisites of Self-Defense

To successfully invoke self-defense, the burden of proof shifts to the accused. They must establish the following three elements by clear and convincing evidence:

I. Unlawful Aggression

This is the condition sine qua non (indispensable requirement). There can be no self-defense without unlawful aggression.

Definition: An actual physical assault or a threat to inflict real injury that is offensive and positively strong.
In Crimes Against Chastity: A woman (or man) defending their honor against an attempted rape is reacting to unlawful aggression. The Supreme Court has long held that "honor" is as precious as life itself; thus, force used to prevent a violation of chastity is legally sanctioned.
Imminence: The danger must be present or actually threatening; it cannot be a past grievance or a future threat.

II. Reasonable Necessity of the Means Employed

The law does not require "perfect" equality between the threat and the response, but it demands reasonableness.

The court considers the nature and quality of the weapon used by the aggressor, the physical condition, size, and age of both parties, and the instinct of self-preservation.
The "Stand Ground" Rule: In the Philippines, one is not required to retreat when attacked. If the person is in a place where they have a right to be, they may stand their ground and repel the aggressor with proportional force.

III. Lack of Sufficient Provocation

The person defending themselves must not have given the aggressor a sufficient reason to attack. If the "defender" provoked the fight, they lose the right to claim self-defense.

2. Self-Defense in Crimes Against Chastity

Crimes against chastity—primarily Rape and Acts of Lasciviousness—carry a unique weight in Philippine jurisprudence.

Defense of Honor: Philippine courts have consistently ruled that a woman being subjected to an attempted rape has the right to use even lethal force if it is the only way to protect her honor and chastity.
The "Kill Rather than Be Defiled" Doctrine: While not an official name, the jurisprudence (e.g., People vs. Luague and Gongot) emphasizes that a woman is not expected to use cool, calculated judgment when her virtue is being violently attacked. If she kills her attacker in the heat of the struggle to avoid being raped, it is generally considered a justified act of self-defense.

3. Defense of Strangers and Relatives

The law extends the right of defense beyond oneself:

Defense of Relatives (Art. 11, Par. 2): You may defend your spouse, ascendants, descendants, or siblings (including in-laws) provided there is unlawful aggression and the means are reasonable. Even if the relative gave provocation, you can still claim this if you were not part of that provocation.
Defense of Strangers (Art. 11, Par. 3): You may defend a complete stranger from a crime against person or chastity. The added requirement here is that the person defending must not be induced by revenge, resentment, or any evil motive.

4. Burden of Proof and the "Admission" Trap

Invoking self-defense is a high-stakes legal strategy. When an accused pleads self-defense, they are effectively admitting to the commission of the act (e.g., "Yes, I killed him") but claiming it was justified.

The Shift: The "presumption of innocence" is effectively neutralized. The accused must now prove the elements of self-defense.
Failure to Prove: If the accused fails to prove any of the three requisites (especially unlawful aggression), they will be convicted of the crime admitted, often with no chance to deny the act later.

5. Summary of Jurisprudential Standards

The Philippine Supreme Court has established that self-defense cannot be based on "mere fear" or a "believed threat" that is not manifest.

Element	Legal Standard
Unlawful Aggression	Must be a physical act or a manifest intent to harm.
Reasonableness	Judged by the "rational necessity" of the action, not mathematical equality.
Retreat	Not required if the person is attacked in a place they have a right to be.
Honor	Protected under the law just as much as physical life.

In cases of crimes against chastity, the law recognizes the psychological trauma and the immediate necessity of defense, often granting wider latitude to the victim in determining what "reasonable force" looked like in the moment of the assault.