Definition
To intervene, in law, carries two distinct meanings that arise in entirely separate contexts.
1. PROCEDURAL INTERVENTION: To formally join an existing lawsuit as a party, despite not having been named by the original plaintiff or defendant. A person or entity intervenes when they have a legal interest in the subject matter of the litigation and seek to protect that interest by participating directly in the proceeding. The person who intervenes is called an intervenor (also spelled intervener). Intervention may be granted as a matter of right — when a statute confers it, or when the intervenor's interest may be impaired by the outcome — or it may be permissive, at the court's discretion, when the intervenor's claim or defense shares a common question of law or fact with the main action.
2. CAUSAL INTERVENTION (TORT LAW): In negligence and tort doctrine, "intervene" describes the act by which a new, independent force enters the causal chain between a defendant's original negligence and a plaintiff's injury. Whether the intervening act breaks the chain of legal causation — and thereby relieves the original defendant of liability — is one of the central problems in tort law. An intervening cause that is sufficiently unforeseeable or independent may become a superseding cause, cutting off liability entirely.
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Common Language
Modern common usage (Wiktionary): To become involved in a situation so as to alter or prevent an action; to come between persons or things; to occur between points in time or events.
Historical common usage (Webster's 1913): To come between persons or things; to occur or fall between events; to interpose for the purpose of settling a dispute.
The ordinary sense of "intervene" — stepping in to prevent or influence something — overlaps with the legal meanings but is too imprecise to guide legal research. In procedural law, intervention is a formal, rule-governed act requiring court permission, not merely the act of getting involved. In tort law, an intervening cause may be entirely passive or mechanical; it need not be a deliberate act of interference. Researchers who import the ordinary meaning risk underestimating the procedural formality required to become an intervenor, and may conflate the neutral causal concept with wrongful or volitional conduct.
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Common Confusion
INTERVENING CAUSE vs. SUPERSEDING CAUSE: These terms are frequently used interchangeably, but they carry different weight in legal analysis. Every superseding cause is an intervening cause, but not every intervening cause is superseding. An intervening cause merely enters the causal sequence after the defendant's negligence; it does not automatically relieve the defendant of liability. A superseding cause is an intervening cause so independent or unforeseeable that it breaks the chain of causation entirely and becomes the legally operative cause of injury. The failure to distinguish between these two concepts is a common error in both advocacy and legal research.
INTERVENTION (PROCEDURAL) vs. JOINDER vs. AMICUS CURIAE: Intervention, joinder, and amicus participation all involve non-original parties appearing in litigation, but they operate differently. Joinder is initiated by an existing party who adds another; intervention is initiated by the outsider seeking to enter. An amicus curiae submits argument to assist the court but does not become a party to the case and acquires no rights in the litigation's outcome. An intervenor, by contrast, does become a party and may be bound by the judgment.
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Core Elements
For procedural intervention, courts typically require the intervenor to establish:
TIMELINESS: The application must be made without undue delay. Courts weigh how far the proceedings have advanced, prejudice to existing parties, and the reason for any delay.
COGNIZABLE INTEREST: The intervenor must assert a legally protectable interest in the property or transaction at stake, not merely a general concern about the outcome.
IMPAIRMENT OF INTEREST: Disposition of the action must threaten to impair or impede the intervenor's ability to protect that interest.
INADEQUATE REPRESENTATION: Existing parties must not adequately represent the intervenor's interest. Where interests are perfectly aligned, intervention as of right may be denied.
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Recognized Forms
/SUBTYPES
INTERVENTION OF RIGHT: Granted when a statute confers an unconditional right to intervene, or when the four-part test above is satisfied. The court has limited discretion to deny it.
PERMISSIVE INTERVENTION: Granted at the court's discretion when the proposed intervenor's claim or defense shares a common question of law or fact with the main action. Courts weigh prejudice and delay.
INTERVENTION AS INTERVENOR-PLAINTIFF or INTERVENOR-DEFENDANT: The intervenor aligns with one side of the existing dispute, asserting claims or defenses accordingly. In some proceedings — particularly class actions, agency review cases, and public interest litigation — the intervenor may assert an independent position aligned with neither original party.
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Why It Matters in Research
In the Law Mind corpus, "intervene" surfaces across at least two independent doctrinal tracks, and conflating them will send a researcher in the wrong direction immediately.
For tort researchers, the critical connection is to superseding cause doctrine. The encyclopedia entry on Negligence — Intervening and Superseding Causes addresses the foreseeability test that determines whether an intervening act cuts off original liability. Historical sources frequently use "intervening cause" as if it automatically defeats liability; modern doctrine does not. Be alert to this shift when reading older treatises or cases.
For procedure researchers, the rules governing intervention have evolved significantly. Equity courts recognized a form of intervention long before modern procedural codes, but the older vocabulary — "petition to intervene," "leave to be heard" — does not map cleanly onto the contemporary rule-based framework. Anderson's treatment reflects the older equity-era conception where intervention was largely discretionary; the modern framework introduced more clearly defined categories of right versus permissive intervention.
Jurisdictional variation matters here. Federal procedure and most state systems recognize the right/permissive binary, but the application of the "adequacy of representation" prong differs meaningfully across circuits and state courts. Public interest and environmental litigation have generated a substantial body of intervention jurisprudence that may not appear in general practice sources.
When researching the causal meaning, watch for historical sources that use "intervening," "independent," and "superseding" interchangeably without distinguishing whether the chain of causation was actually broken. The terminology stabilized later than practitioners sometimes assume.
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Historical Dictionary Support
Anderson's Dictionary of Law provides the clearest historical foundation: "To file a claim or a defense in a suit instituted by or against others," with the intervenor described as one who "applies to be heard as an original party in another's suit, he being interested in the result of the suit." This captures the procedural core accurately, though the framing is equity-era and does not distinguish intervention of right from permissive intervention — a distinction that would become structurally important under codified civil procedure.
The Rapalje & Lawrence entry in the source material does not yield a usable definition for this term; the extracted text appears to be misdirected to an unrelated subject. Researchers should treat that source as silent on this entry.
Neither historical dictionary addresses the causal meaning of intervene as it operates in tort doctrine — a gap that reflects the later systematization of negligence law. For that dimension of the term, the historical dictionaries are not useful starting points.
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Jurisdictional Note
Federal courts apply a four-part test for intervention of right that has been refined through circuit court decisions; circuits have divided notably on the standard for "adequacy of representation," particularly in cases involving government defendants and public interest organizations seeking to intervene. State courts vary in whether they follow the federal structure closely or apply independent frameworks derived from equity practice or local procedural codes.
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