In the common law, legal fictions, are suppositions of fact taken to be true by the courts of law, but which are not necessarily true. They typically are done to evade archaic rules of procedure or to extend the jurisdiction of the courts in ways that were considered useful, but not strictly authorised by the old rule.

A simple legal fiction extended the jurisdiction of the court of the Exchequer, in England, to all manner of cases involving debt. The Exchequer was originally a court whose specialized jurisdiction involved taxes and other obligations to the Crown, and which had only slight jurisdiction over private matters between litigants. The Exchequer therefore had a much lighter caseload than the King's Bench and other courts in England. Those who commenced an action in the Exchequer on a debt, therefore, had to plead that they owed money to the King, but that they could not pay it because the debtor wrongfully withheld payment. It came to pass that the debt owed to the King became a legal fiction, in that the debtor was not entitled to controvert this allegation, be it true or false, in order to oust the Exchequer from jurisdiction. By this artifice the creditor could bring his case in a court with a much less crowded docket. A similar but more complicated legal fiction involved pleadings in cases where title to real property was tried. The common law had a procedure whereby title to land could be put in direct issue, called the writ of right. One inconvenience of this procedure, though, was that the defendant at his option could insist on trial by wager of battel, which is to say, trial by combat, a judicially sanctioned duel. Most plaintiffs were unwilling to stake life and limb on the hazard of the battle, so the procedure fell into disuse. Rather, an elaborate tale was told in the pleadings, about how one John Doe leased land by the plaintiff, but that he was ousted by Richard Roe, who claimed a contrary lease from by the defendant. These events, if true, led to the assize of novel disseisin, later called the mixed action in ejectment, a procedure in which title could ultimately be determined, but which led instead to trial by jury. This is the origin of the names John Doe, Richard Roe, and so forth, for anonymous parties. The fiction of Doe, Roe, and the leases was not challenged by the parties unless they wished to stake their life and safety on a trial by combat. Wager of battel was in fact not abolished in England until 1819.

Another legal fiction involves resignation from Parliament. In 1623 a rule was declared that said that members of Parliament were given a trust to represent their constituencies, and therefore were not at liberty to resign them. In those days, Parliament was relatively weaker, and service was sometimes considered a resented duty rather than a position of power and honour. However, an MP who accepts an "office of profit" from the Crown was obliged to leave his post, it being feared that his independence was compromised if he be in the King's pay. Therefore, the device was invented that the MP who wished to quit applied to the King for the post of "steward of the Chiltern Hundreds", an obsolescent office of negligible duties and scant profit, but an office in the King's gift nonetheless. The first MP to avail himself of the Chiltern Hundreds to leave Parliament was John Pitt in 1751.

A rather significant legal fiction that is still extant today is the concept that a corporation is a person (see Corporate personhood). In the common law tradition, only a person could sue or be sued. This was not a problem in the era before the Industrial Revolution, when the typical business venture was a sole proprietorship- the proprietor was simply liable for the debts of the business. A feature of the corporation, however, is that the owners/shareholders enjoyed limited liability- the owners were not liable for the debts of the company. Courts created an elegant solution- a corporation is a person, and could therefore sue and be sued, and thus held accountable for its debts. This ensured that creditors would be able to seek relief in the courts should the corporation default on its obligations, encouraging banks to extend credit to the corporation. This simple fiction enabled corporations to acquire vast wealth, grow in scope, and become the preferred organization for businesses of all sizes.

Legal fictions are fewer in number than they used to be. The elaborate pantomime about poor Doe left homeless by Roe has been abolished by statute or by reforms in civil procedure in every common law jurisdiction. The business about Doe and Roe being the guardians of undisclosed parties who wish to bring suit, or the names of parties unknown, remains in some jurisdictions (although not in England). Also, legal fictions have been invalidated as being contrary to public policy. An example of this is the Australian High Court's rejection in the Mabo cases of the doctrine of terra nullius which was the legal fiction that there were no property rights in land in Australia before the time of European colonization.

Some have argued that legal fictions seem a baroque excrescence on the law that ought to be excised by legislation. This idea occurs to many who first encounter the notion that the law entertains fictions. Jeremy Bentham sharply criticised the notion of legal fictions, saying that "fictions are to law what fraud is to trade."

In their defence, most legal fictions were harmless vestiges of history whose traces may be worth preserving for their own sake. William Blackstone defended them, observing that legislation is never free from the iron law of unintended consequences. Using the metaphor of an ancient castle, Blackstone invoked the metaphor that:

We inherit an old Gothic castle, erected in the days of chivalry, but fitted up for a modern inhabitant. The moated ramparts, the embattled towers, and the trophied halls, are magnificent and venerable, but useless. The interior apartments, now converted into rooms of convenience, are chearful and commodious, though their approaches are winding and difficult.

The past preserved by legal fictions has its own value, which it may be unwise to sweep away in the interest of streamlined simplicity.