Legal Maxims and Phrases in UK Law: A Practical Guide for New Professionals

What are legal maxims?

Short, often Latin, statements expressing fundamental legal principles. They summarise rules that underpin statutory and case law, and they persist because they compress a complex principle into something memorable.

They are not a source of law. A court decides a case on statute and authority, and a maxim is a convenient label for what the authority establishes. That distinction matters because maxims are stable while the law they label is not: the words survive unchanged while the underlying rule is qualified, narrowed or replaced.

Criminal law maxims

Actus non facit reum nisi mens sit rea

An act does not make a person guilty unless the mind is also guilty. Criminal liability generally requires both a prohibited act and the relevant mental state. The qualification matters: strict liability offences, common in regulatory and road traffic contexts, dispense with the mental element entirely.

Nullum crimen sine lege

No crime without law. Conduct cannot be criminal unless it was prohibited at the time it occurred, which is the principle behind the prohibition on retrospective criminal liability.

Actori incumbit onus probandi

The burden of proof lies on the party asserting. In criminal proceedings the burden rests on the prosecution, subject to limited statutory exceptions, a principle expressed by Viscount Sankey in Woolmington v DPP [1935] AC 462 as the golden thread running through English criminal law.

Worth noting for anyone who has seen it in a textbook: in dubio pro reo, where doubt favours the accused, is a civil law formulation rather than an English maxim. The English equivalent is the criminal standard of proof and the burden as stated in Woolmington.

Natural justice and procedural maxims

Audi alteram partem

Hear the other side. A person affected by a decision must have a fair opportunity to answer the case against them. Ridge v Baldwin [1964] AC 40 remains the modern foundation: a Chief Constable dismissed without a proper opportunity to respond had the decision quashed, and the case re-established natural justice as a general requirement of administrative decision-making.

Nemo judex in causa sua

No one should be a judge in their own cause. The rule against bias covers both actual bias and the appearance of it, and it reaches interests that are not financial. In R v Bow Street Metropolitan Stipendiary Magistrate, ex parte Pinochet Ugarte (No 2) [2000] 1 AC 119, the House of Lords set aside its own earlier decision because of a judge’s connection to an intervening party.

Fiat justitia ruat caelum

Let justice be done though the heavens fall. Rhetorical rather than operative, and better suited to a speech than a skeleton argument.

Tort law maxims

Res ipsa loquitur

The thing speaks for itself. Where an accident is of a kind that does not ordinarily happen without negligence, and the thing causing it was under the defendant’s control, negligence may be inferred without direct evidence of what went wrong. The classic statement is in Scott v London and St Katherine Docks Co (1865), and it is best understood as an evidential inference rather than a separate legal doctrine.

Volenti non fit injuria

No injury is done to one who consents. A claimant who freely and knowingly accepted the risk may be unable to recover. The defence is narrow in practice, requiring genuine voluntary acceptance of the specific risk rather than mere knowledge of it, and it is heavily restricted by statute in employment and road traffic contexts.

Lex loci delicti

The law of the place where the wrong occurred. The starting point for choice of law in tort, though the applicable rules in cross-border cases are statutory and subject to displacement, so the maxim states a default rather than an answer.

Contract and commercial maxims

Pacta sunt servanda

Agreements must be kept. The foundational principle of contractual obligation, and the reason the exceptions, frustration, duress, misrepresentation, illegality, have to be established rather than assumed.

Ex turpi causa non oritur actio

No action arises from a dishonourable cause. This is the maxim most often stated in an outdated form, and it is worth getting right.

The traditional gloss, that no claim arises from an illegal act, is too blunt. In Patel v Mirza [2016] UKSC 42, a nine-justice Supreme Court departed from the reliance test in Tinsley v Milligan and replaced it with a structured assessment of public policy. The question is now whether denying the claim would be a proportionate response to the illegality, having regard to the purpose of the prohibition, any countervailing public policies, and the integrity of the legal system. Mr Patel recovered £620,000 paid under an agreement that amounted to a conspiracy to commit insider dealing. Judgment.

A trainee who states the maxim in its pre-2016 form is describing a test the Supreme Court has expressly abandoned.

Caveat emptor

Let the buyer beware. Still meaningful in some commercial and property contexts, where the buyer is expected to investigate, but substantially displaced in consumer transactions by statutory implied terms and remedies under the Consumer Rights Act 2015. Stating it without that qualification misdescribes the modern position for most retail sales.

The counterpart sometimes offered, caveat venditor, is not a traditional English legal maxim. It is a modern commentary coinage describing the effect of consumer protection legislation, and it should not be cited as though it were established.

Equitable maxims

Equity developed its own body of maxims, and these are still cited in argument because equitable remedies remain discretionary. A claimant asking for an injunction, specific performance or rescission is asking the court to exercise a discretion, and these maxims describe how that discretion is exercised.

He who comes to equity must come with clean hands

A claimant whose own conduct in the matter has been improper may be refused equitable relief. The misconduct must relate to the subject of the claim rather than to the claimant’s character generally, which is the limit courts apply when the argument is stretched too far.

He who seeks equity must do equity

Relief may be conditioned on the claimant fulfilling their own obligations. The distinction from clean hands is one of time: clean hands looks backwards at past conduct, this looks forward at what the claimant must do to obtain the remedy.

Delay defeats equity

The doctrine of laches. Unreasonable delay in asserting an equitable right, particularly where it has prejudiced the other party, may bar relief. It operates alongside statutory limitation rather than instead of it, and can defeat a claim brought within the limitation period.

Equity looks to intent rather than form

Substance prevails over labels. A transaction is characterised by what the parties actually did and intended, not by what they called it, which is why a document described as a licence may be held to create a tenancy.

Equity acts in personam

Equitable orders bind the person rather than operating directly on property, which is what allows the court to make orders affecting assets outside the jurisdiction where it has personal jurisdiction over the defendant.

Where the equities are equal, the first in time prevails

A priority rule between competing equitable interests, subject to the registration regimes that now govern most priority questions in land.

Equity will not assist a volunteer

A person who has given no consideration generally cannot compel completion of an incomplete gift, subject to well-established exceptions.

Property and commercial maxims

Nemo dat quod non habet

No one gives what they do not have. A seller cannot transfer better title than they possess, expressed in section 21 of the Sale of Goods Act 1979: where goods are sold by someone who is not the owner, the buyer generally acquires no better title than the seller had.

The exceptions are where the practical work happens: sale under a voidable title, seller or buyer in possession, dispositions by mercantile agents under the Factors Act 1889, and the protection given to private purchasers of vehicles subject to hire purchase. The maxim states the rule; the exceptions decide most disputes.

Contra proferentem

An ambiguous clause is construed against the party who put it forward. Its role has narrowed considerably: courts are reluctant to strain clear language in commercial contracts between parties of comparable bargaining power, and statutory controls now do most of the work on exclusion clauses in consumer contexts. Treat it as a tie-breaker for genuine ambiguity rather than a general principle of construction.

Quicquid plantatur solo, solo cedit

Whatever is attached to the soil becomes part of it. The starting point for fixtures, determined in practice by the degree and purpose of annexation rather than by the maxim alone.

Consensus ad idem

Agreement on the same thing. The requirement that parties reach a genuine meeting of minds, assessed objectively by what a reasonable person would understand from their words and conduct rather than by their private intentions.

Latin still in daily use

Separately from maxims, a set of Latin terms remains standard vocabulary in English practice and has not been displaced by the plain English reforms. These describe how the system works rather than stating principles, and trainees meet them constantly.

These are safe to use with a professional audience. The test is whether the reader will understand it faster than the English equivalent, and for this set they usually will.

General principles

Ignorantia juris non excusat

Ignorance of the law is no excuse. A defendant cannot escape liability by showing they did not know the conduct was prohibited, though mistake of fact operates differently and can be relevant to the mental element of an offence.

Ubi jus ibi remedium

Where there is a right, there is a remedy. Aspirational as much as descriptive: the law recognises rights for which practical remedies are limited, and the maxim is better used to justify a remedy than to assert one exists.

De minimis non curat lex

The law does not concern itself with trifles. Used to dispose of claims and breaches too trivial to warrant the court’s attention, and frequently invoked in costs and proportionality arguments.

Qui facit per alium facit per se

He who acts through another acts himself. The principle behind agency and, in modified form, vicarious liability.

Salus populi suprema lex

The welfare of the people is the supreme law. A statement of political philosophy rather than a rule courts apply, and it carries little weight as a submission.

Statutory interpretation maxims

These are the maxims most likely to be genuinely useful in practice, because courts still reason with them explicitly.

Expressio unius est exclusio alterius. The express mention of one thing excludes others. Where a statute lists specific items, the omission of a comparable item may be taken as deliberate.

Noscitur a sociis. A word is known by the company it keeps. An ambiguous term takes colour from the words around it.

Ejusdem generis. Of the same kind. Where general words follow a list of specific ones, the general words are read as limited to the same class.

All three are presumptions rather than rules, displaced where the statutory purpose points the other way.

Why civil procedure abandoned its Latin

One reason to be careful with Latin is that civil procedure has spent twenty-five years removing it.

The Woolf reforms that produced the Civil Procedure Rules deliberately replaced procedural Latin and archaic terms with plain English, on the view that litigants should be able to understand the process they are in. A writ became a claim form, a plaintiff became a claimant, an application ex parte became one made without notice, a hearing in camera became one in private, a subpoena became a witness summons, and a guardian ad litem became a litigation friend.

The point for a trainee is practical. Using superseded procedural Latin in a document signals unfamiliarity with the current rules rather than erudition, and in correspondence with a litigant in person it is close to obstructive. Our guide to the Civil Procedure Rules covers the framework that replaced it.

Four mistakes to avoid

The errors that get noticed are rarely about pronunciation.

Using a maxim as authority. A skeleton argument that asserts a principle by maxim and cites nothing has asserted nothing. Name the case or the section, and let the maxim label it.

Relying on the traditional formulation. Ex turpi causa and caveat emptor are the two most commonly quoted in forms the law has moved past. Before using either, check what the current test actually is.

Overreaching with equitable maxims. Clean hands in particular gets stretched into an argument that the other side is generally unmeritorious. The misconduct has to relate to the subject matter of the claim, and an argument that ignores that limit invites the judge to say so.

Using superseded procedural Latin. Writ, plaintiff, ex parte and in camera were deliberately replaced. Using them in correspondence with a court or a litigant in person reads as unfamiliarity with the rules rather than expertise.