Answer
Self-defense is one of the justifying circumstances under Article 11 of the Revised Penal Code. The Supreme Court has consistently held that it has three requisites: (1) unlawful aggression on the part of the victim; (2) reasonable necessity of the means employed to prevent or repel the aggression; and (3) lack of sufficient provocation on the part of the person defending himself.
Unlawful aggression is the indispensable requirement — without an actual or imminent unlawful attack on the accused's life or safety, there is nothing to repel and self-defense cannot be appreciated at all. Because self-defense is a justifying circumstance, an accused who invokes it admits the killing or injury but is held not criminally liable; the burden then shifts to him to prove the three requisites by clear and convincing evidence.