The right of self-defense (also called, when it applies to the defense of another, alter ego defense, defense of others, defense of a third person) is the right for people to use reasonable or defensive force, for the purpose of defending one's own life (self-defense) or the lives of others, including – in certain circumstances – the use of deadly force.

If a defendant uses defensive force because of a threat of deadly or grievous harm by the other person, or a reasonable perception of such harm, the defendant is said to have a "perfect self-defense" justification. If the defendant uses defensive force because of such a perception, and the perception is not reasonable, the defendant may have an "imperfect self-defense" as an excuse.

General concepts – legal theory.

Justification does not make a criminal use of force lawful; if the use of force is justified, it cannot be criminal at all.

The early theories make no distinction between defense of the person and defense of property. Whether consciously or not, this builds on the Roman Law principle of dominium where any attack on the members of the family or the property it owned was a personal attack on the pater familias – the male head of the household, sole owner of all property belonging to the household, and endowed by law with dominion over all his descendants through the male line no matter their age. The right to self-defense is phrased as the principle of vim vi repellere licet ("it is permitted to repel force by force") in the Digest of Justitian (6th century). Another early application of this was Martin Luther's concept of justified resistance against a Beerwolf ruler, which was used in the doctrine of the lesser magistrate propounded in the 1550 Magdeburg Confession.

In Leviathan (1651), Hobbes (using the English term self-defense for the first time) proposed the foundation political theory that distinguishes between a state of nature where there is no authority and a modern state. Hobbes argues that although some may be stronger or more intelligent than others in their natural state, none are so strong as to be beyond a fear of violent death, which justifies self-defense as the highest necessity. In the Two Treatises of Government, John Locke asserts the reason why an owner would give up their autonomy:

...the enjoyment of the property he has in this state is very unsafe, very insecure. This makes him willing to quit a condition, which, however free, is full of fears and continual dangers: and it is not without reason, that he seeks out, and is willing to join in society with others, who are already united, or have a mind to unite, for the mutual preservation of their lives, liberties and estates, which many call by the general name, property.

In earlier times before the development of national policing, an attack on the family home was effectively either an assault on the people actually inside or an indirect assault on their welfare by depriving them of shelter and or the means of production. This linkage between a personal attack and property weakened as societies developed but the threat of violence remains a key factor. As an aspect of sovereignty, in his 1918 speech Politik als Beruf (Politics as a Vocation), Max Weber defined a state as an authority claiming the monopoly on the legitimate use of physical force within defined territorial boundaries. Recognizing that the modern framework of nations has emerged from the use of force, Weber asserted that the exercise of power through the institutions of government remained indispensable for effective government at any level which necessarily implies that self-help is limited if not excluded.