In a breach of contract dispute, there are certain general things a plaintiff must prove:

• A contract exists.
• The plaintiff satisfied his part of the contract.
• The defendant did not satisfy his part of the contract.
• The defendant’s non-compliance with the contract has caused quantifiable losses (damages) to the plaintiff.

Most breach of contract defenses invoke what are known as an “affirmative defense”—a defense that does not dispute the plaintiff’s primary claims or facts, but that raises issues that show why the contract should not be enforced. Essentially, the defendant is giving reasons why the plaintiff should not win, even if what the plaintiff is saying is true.

If the dispute goes to trial, the burden of proof in an affirmative defense falls upon the defendant—he must prove why the contract should not be enforced. There are many potential arguments a defendant can use. The defense may fall within one (or more) of these categories:

• The contract is illegal. A contract is unenforceable if the item or service being contracted for is itself illegal. A contract to commit murder, for example, would be unenforceable, and a defendant would not be expected to satisfy the terms of that contract. While murder is an extreme example, there may be many other statutory reasons that would make the object of a contract illegal.
• The contract violates public policy. Similar to the above argument, this defense argues that enforcement of the contract would violate public policy and would harm society as a whole. For example, a contract involving sexual favors would likely be determined to run afoul of “public sensibilities” and thus be unenforceable.
• The contract is impossible or impractical. In some cases, it may simply be impossible to carry out the terms of a contract as written. (Example: A painter contracts with a homeowner to paint a house, but then the house burns down.)
• A party entered into the contract under duress or undue influence. If one party was forced to enter the contract due to fraud or undue influence, the contract may be ruled unenforceable. For example, if the defendant signed the contract after being threatened with physical harm, the contract would most likely be deemed unenforceable.
• A party lacked capacity to enter the contract. All parties in a contract must understand the implications of the agreement. If one party was mentally unfit, for example, or not of legal age, the contract may be judged voidable.
• The contract is unconscionable or grossly unfair. If the plaintiff was in a much stronger bargaining position than the defendant, and the terms of the contract are overwhelmingly beneficial to the plaintiff, the contract may be ruled unconscionable or grossly unfair.
• The contract contains mistakes. Mistakes may be mutual—made by both parties in the contract—or unilateral—made by only one party. Usually, a mutual mistake, such as an incorrect date, is more likely to void a contract than a unilateral one.

These are general principles, and there are many other specific defenses a party to a contract might use. Under the principle of “Arguments in the Alternative,” a defendant may make more than one of these arguments simultaneously—even if they contradict each other. The court must consider each argument separately from the others. For example, a defendant can put forth two arguments at the same time: that he did in fact satisfy the terms of the contract, and that he was justified in not satisfying the terms of the contract. Even though the arguments cannot both be true, both will be considered individually, irrespective of the other.