Definition
Physical necessity is a condition in which a person is compelled to act — or to refrain from acting — by an overwhelming external force that leaves no realistic alternative. The compulsion is external and physical in character, not merely moral, psychological, or situational. Because the actor has no genuine choice, physical necessity is treated in law as negating the voluntariness of the act, which bears directly on liability in criminal law, tort law, and contract law alike.
Physical necessity appears across several distinct legal contexts:
1. Criminal law. Physical necessity is invoked as a complete defense when a defendant's conduct was compelled by physical forces beyond human resistance. It differs from the broader necessity defense (choice of evils) in that physical necessity focuses on the absence of any voluntary act rather than on a justification for choosing between competing harms. A person physically forced to pull a trigger by another's hand has not committed a voluntary act at all; a person who shoots to prevent a greater harm has committed a voluntary act but seeks justification.
2. Tort law. Where a defendant acts under physical necessity, the element of voluntariness required for most intentional tort claims may be absent. The defense is distinct from the necessity privilege, which is a justification doctrine rather than a claim that the act was involuntary.
3. Property law — easements by necessity. Physical necessity gives rise to implied easements where land is landlocked and access to a public road is physically impossible without crossing a neighbor's parcel. This use of the term is functional and contextual; the necessity is geographic and practical, not a defense to liability.
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Common Language
Modern common usage (Wiktionary): "Necessity" in ordinary English means the state of being required or unavoidable; something indispensable. "Physical" denotes relating to the body or to material things, as opposed to mental or spiritual.
Historical common usage (Webster's 1913): Necessity — "the quality or state of being necessary, unavoidable, or absolutely requisite." Physical — "of or pertaining to nature or to natural or material things."
The compound phrase "physical necessity" in ordinary English suggests any unavoidable practical constraint — financial hardship, geographic limitation, or bodily incapacity might all colloquially qualify. The legal meaning is narrower and more demanding: it requires overwhelming external compulsion that destroys the voluntariness of the act itself, not merely circumstances that made a different choice difficult or unlikely.
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Common Confusion
Physical necessity is frequently conflated with moral necessity and with the broader necessity defense (choice of evils). The distinctions matter for research:
Physical necessity vs. moral necessity. Black's draws the line explicitly. Moral necessity arises when a rational being has a duty to perform an act — the compulsion is normative, not physical. A soldier obligated by oath to follow orders acts under moral necessity; a person whose arm is seized and forced to strike another acts under physical necessity. Historical sources treat these as distinct concepts, but older case law sometimes uses "necessity" loosely to cover both.
Physical necessity vs. the necessity defense. The necessity defense (choice of evils) presupposes a voluntary act and asks whether that act was justified by the need to prevent a greater harm. Physical necessity, strictly applied, removes the act from the voluntary category entirely. Researchers will encounter both doctrines cited as "necessity" in older opinions without clear differentiation; the context and the court's framing determine which doctrine is actually at work.
Physical necessity vs. compulsion/duress. Duress involves a human threat that overcomes the will; physical necessity involves an external force that overrides the body. The legal consequences can overlap, but the analytical frameworks are different.
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Why It Matters in Research
The primary research trap with physical necessity is terminological collapse. Nineteenth- and early twentieth-century opinions — and many treatises — use "necessity" as a generic term covering physical necessity, moral necessity, the choice-of-evils justification, and economic or practical compulsion without consistent differentiation. A researcher mining historical case law for physical necessity doctrine must read surrounding context carefully before concluding that a court applied the concept in its strict sense.
In criminal law sources, watch for the transition in mid-twentieth century scholarship and codification (particularly after the Model Penal Code's influence) toward treating involuntary act and necessity as separate doctrinal inquiries. Pre-MPC sources may handle both under a single necessity heading.
In property law, physical necessity for easement purposes carries its own distinct body of doctrine and should be researched through property headings, not criminal or tort headings. The phrase is the same; the doctrine is separate.
Jurisdictional variation in the easement-by-necessity context is particularly sharp: some states require strict physical landlocking; others accept practical necessity. Ensure you are working in the right doctrinal lane before cross-referencing cases.
The Black's citation to "3 Sum. 248" refers to a volume of Justice Story's circuit court reports (Sumner's Reports), indicating the definition's antebellum vintage. Researchers should treat it as an early framing that predates modern actus reus analysis and the codified necessity defense.
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Historical Dictionary Support
Black's Law Dictionary defines physical necessity as "a condition in which a person is absolutely compelled to act in a particular way by overwhelming superior force," and contrasts it with moral necessity, which "arises where there is a duty incumbent upon a rational being to perform, which he ought at the time to perform." This formulation, drawn from Story's circuit reports, has been stable across editions in its core structure, though modern editions typically integrate it into broader discussions of necessity and voluntariness rather than presenting it as a freestanding entry.
The historical sources are thin on physical necessity as an independent topic. Most major legal dictionaries and treatises subsume it under necessity generally or under the voluntariness element of actus reus without giving it a discrete doctrinal treatment. What the historical record shows clearly is that the physical/moral distinction was recognized early and treated as legally significant — but the case law working through that distinction developed unevenly, and the terminology was not always applied with precision.
Bouvier's Law Dictionary entries on necessity similarly blend physical and moral compulsion without sharp separation, which reflects the state of doctrine in the nineteenth century more than a deliberate analytical choice.
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Jurisdictional Note
Physical necessity as a complete defense to criminal liability is recognized across common law jurisdictions but is rarely codified as a distinct doctrine — most modern statutes subsume it under voluntary act requirements or the general necessity defense. In property law, the threshold of physical necessity required to create an easement by implication varies meaningfully by state: strict necessity jurisdictions require actual landlocking, while others allow the showing of reasonable necessity. Research in any specific jurisdiction should confirm which standard applies.
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Encyclopedia Cross-Reference
The Law Mind Criminal Law Encyclopedia — Necessity (Choice of Evils): For the relationship between physical necessity and the broader choice-of-evils justification doctrine.
The Law Mind Torts & Personal Injury Encyclopedia — Intentional Torts — Persons — Necessity as a Defense (Public and Private): For physical necessity as it bears on voluntariness and the necessity privilege in tort.
The Law Mind Property Law Encyclopedia — Easements — Creation by Implication (Prior Use, Necessity): For physical necessity as the threshold requirement for landlocked-parcel easements.
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