Definition
To intervene is to enter into a pending legal proceeding as a party, where one was not originally named as plaintiff or defendant, in order to assert or protect an interest affected by that proceeding. The person who intervenes is the intervener (also spelled intervenor).
The term carries two distinct legal meanings depending on context:
1. PROCEDURAL INTERVENTION (Civil Procedure): A third party's formal entry into an existing lawsuit. The intervener claims a legal interest in the subject matter of the litigation — or in the property or transaction at issue — and seeks to protect that interest by becoming a party rather than merely appearing as a witness or amicus. Intervention may be as of right (where the claim of interest is strong enough that exclusion would impair it) or permissive (where the court exercises discretion to allow entry when the intervener's claim and the main action share a common question of law or fact).
2. TORTIOUS/CAUSAL INTERVENTION (Tort Law): An act or force that comes between a defendant's original negligent conduct and the plaintiff's injury. An intervening cause is a new, independent act or event that contributes to the harm. Whether the intervening cause breaks the chain of legal causation — becoming a superseding cause that relieves the original defendant of liability — depends on foreseeability and the nature of the intervening act.
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Common Language
Modern common usage (Wiktionary): To intervene is to come between parties in a dispute or situation in order to affect the outcome; to intercede or interfere.
Historical common usage (Webster's 1913): "To come between; to occur, fall, or come in between points of time, or events; to interpose." Webster's also notes the sense of occurring between two things as a separating or modifying event.
The common meaning captures something real about both legal senses — entry between existing parties, or an event falling between cause and effect — but it misses the procedural formality of civil-procedure intervention and the specific doctrinal consequences of causal intervention in tort law. A researcher reading "intervene" in an older opinion must determine at the outset which sense the court is using, since the consequences are entirely different.
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Common Confusion
INTERVENING CAUSE vs. SUPERSEDING CAUSE: These are not synonyms. Every superseding cause is first an intervening cause, but not every intervening cause is superseding. An intervening cause merely enters the causal chain; it does not automatically defeat the original defendant's liability. A superseding cause is an intervening cause so unforeseeable, independent, or extraordinary that it severs the defendant's legal responsibility entirely. Historical sources often use "intervening cause" loosely to mean what modern doctrine calls a superseding cause. Researchers reading older materials must verify which concept is actually in play.
INTERVENER vs. AMICUS CURIAE: An intervener becomes a party and is bound by the judgment. An amicus curiae appears to advise the court but is not a party and acquires no rights through the proceeding. The distinction matters enormously for appeal rights and preclusion.
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Recognized Forms
/SUBTYPES
Intervention as of Right: Entry that must be permitted when the intervener claims an interest relating to the property or transaction in dispute, and disposing of the action may practically impair that interest, and existing parties do not adequately represent it.
Permissive Intervention: Entry at the court's discretion, typically where the intervener's claim or defense and the main action share a common question of law or fact. Courts weigh undue delay and prejudice to existing parties.
Intervening Cause (Tort): A cause that enters the causal sequence between defendant's conduct and plaintiff's injury. May or may not break the chain of liability.
Superseding Cause (Tort): A form of intervening cause sufficient to relieve the original tortfeasor of liability because the intervening act was unforeseeable or independent enough to be treated as the sole proximate cause.
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Why It Matters in Research
The two meanings of "intervene" occupy almost entirely separate doctrinal literatures, yet the word itself appears in both without flag or signal. A researcher in procedural materials is navigating party-joinder, standing, and judgment-binding rules. A researcher in tort materials is navigating proximate causation, foreseeability, and liability apportionment. Conflating them produces genuine analytical error.
In historical sources, the procedural sense appears primarily in equity practice, where intervention as a device for protecting third-party interests was well developed before the merger of law and equity. Post-merger procedural codes (and eventually the Federal Rules) formalized what equity practice had recognized by custom. Older equity opinions may use "intervene" without statutory grounding — the court was exercising inherent equitable jurisdiction. Researchers must not assume modern Rule 24-style criteria apply to pre-merger materials.
In tort materials, the causal sense of "intervene" becomes particularly treacherous in older sources because the vocabulary was not standardized. Courts before the mid-twentieth century sometimes used "intervening cause," "independent cause," "efficient cause," and "superseding cause" interchangeably. The doctrinal split between merely intervening causes (which preserve original liability) and superseding causes (which defeat it) was not cleanly articulated in many jurisdictions until the Restatement era. Any historical tort opinion using "intervening" to discuss causation should be read with care about which modern concept is actually being applied.
The Law Mind corpus connects these two meanings across different encyclopedias and treatise sections. Do not assume that a cross-reference to "intervening" in a procedural context has anything to do with the causation doctrine, and vice versa.
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Historical Dictionary Support
Rapalje & Lawrence define intervener as "a person who intervenes or interposes; one who, not being originally a party to a suit, but claiming an interest in the subject-matter in dispute, is permitted to make himself a party to the suit." The definition is procedural in focus and reflects the equity tradition clearly — the emphasis on "claiming an interest" and being "permitted" to join signals the discretionary, court-supervised nature of intervention as Rapalje & Lawrence knew it.
Rapalje & Lawrence do not address the causal/tort sense of "intervening" as a term of art, which reflects both the era's less developed proximate-cause vocabulary and the subject-matter focus of a dictionary written before modern tort doctrine systematized the intervening/superseding distinction. Researchers relying solely on Rapalje & Lawrence for the causal sense will find the source silent — the gap is informative rather than troubling.
Neither definition in Rapalje & Lawrence distinguishes intervention as of right from permissive intervention, a distinction that became formal with the adoption of procedural codes and eventually the Federal Rules of Civil Procedure. Historical practice recognized gradations of interest, but not the binary of right versus permissive as modern doctrine frames it.
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Jurisdictional Note
Procedural intervention standards vary between federal and state courts, and among states. Federal practice under Rule 24 distinguishes intervention as of right from permissive intervention with specific criteria for each. Many states have analogous but not identical rules; some retain more equity-influenced discretionary standards. In any jurisdiction, the threshold question — whether an interest is legally sufficient to support intervention as of right — is contested and fact-sensitive.
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Encyclopedia Cross-Reference
Law Mind Torts & Personal Injury Encyclopedia — Negligence: Intervening and Superseding Causes (torts_15): Essential reading for the causal sense of "intervene." Covers the doctrinal distinction between intervening and superseding causes, foreseeability analysis, and the conditions under which an intervening act breaks the chain of proximate causation.
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