Even those plaintiffs who establish a prima facile case are not automatically entitled to compensation. Defendants may try to justify their conduct through the use of various defenses. This means that even if a plaintiff proves all of the elements of her claim, a defendant can avoid liability by proving a different set of facts to support the defense.

1.     Consent

When a person voluntarily and willfully agrees to undertake an action that another person suggests, the consenting person must possess sufficient mental capacity. Consent is a defense to an intentional tort claim. The defense derives from the common law principle volenti non fit injuria (自愿承担风险) – to one who is willing, no working is done. For example, if you point your nose and tell your classmate to “go ahead and hit me,” your will not be able to maintain a battery action of he accepts your invitation.

But consent extends beyond verbal expressions. For example, a plaintiff may communicate consent by nations as well as words. The famous case of O’ Brien v. Cunard Steamship Co..

 

2.     Self Defense

According to the Restatement of Torts:

An actor is privileged to use reasonable force, not intended or likely to cause death or serious bodily harm, to defend himself against unprivileged harmful or offensive contact or other bodily harm which he reasonable believes that another is about to inflict intentionally upon him.

 

3.     Defense of Property

The question of when an individual can engage in otherwise tortious activity to protect is similar to the question of when an individual can do so in self defense. In general, a person may use reasonable force to protect property when she reasonably believes that force is necessary to prevent the intrusion.

The issue of what constitutes reasonable force sometimes poses problems. As with self defense, the use of force must be proportional to the threatened interest. Proportionality generally does not exist, however, when a person uses deadly force to protect property the alone. For example, the famous case of Bird v. Holbrook.

 

4.     Necessity

In tort law,  there are two different categories of the necessity defense that can be employed: public necessity and private necessity.

4.1  Private Necessity

The law permits interference with another’s property in certain emergency situation. Courts and commentators divide this “necessity” privilege into two categories—public  necessity and private necessity provide defendants with  an absolute privilege to interfere with the property of others to avoid a “ public disaster.” Private necessity, on the other hand, provides defendants with a qualified privilege to interfere with property  to protect their own interests, or those of a small group of others. Private necessity is a qualified privilege because it still requires the defendant to compensate the property owner for any damage caused. 

4.2  Public Necessity

The Restatement of Torts provide:

One is privileged to commit an act which would otherwise be a trespass to a chattel or a conversion if the act is or is reasonable believed to be necessary for the purpose of avoiding a public disaster.

As noted in the introduction to this section, the most important distinction between public necessity and private necessity is that public necessity is an absolute privilege—in other words, a defendant who successfully raises the privilege need not compensate the plaintiff for his loss.