Definition
Conformable to reason; just, fair, and proportionate under the circumstances. In law, "reasonable" functions less as a fixed standard and more as a relational judgment — what is reasonable depends on the context in which the question is asked, the nature of the obligation at issue, and the position of the person whose conduct is being evaluated.
Because "reasonable" appears across virtually every field of law, its meaning shifts substantially by context:
1. REASONABLE AS A CONDUCT STANDARD. In tort law, negligence, and criminal procedure, "reasonable" describes the conduct expected of an ordinary, prudent person in similar circumstances. It is an objective, external measure — not what this particular actor believed, but what a hypothetical reasonable person would have done. This is the most litigated usage of the term.
2. REASONABLE AS A TEMPORAL STANDARD. "Reasonable time" appears throughout contract law wherever parties have not specified a deadline. Courts supply the term by implication, asking what time a person of ordinary prudence would require given the nature of the transaction and surrounding circumstances.
3. REASONABLE AS A PROPORTIONALITY STANDARD. In fee awards, constitutional law, and administrative law, "reasonable" functions as a ceiling on excess — a fee, a condition, a restriction, or a restraint must not be more burdensome than the purpose warrants.
4. REASONABLE AS A PROBABILITY THRESHOLD. "Reasonable cause," "reasonable suspicion," and "reasonable belief" each describe points on an evidentiary or probabilistic spectrum — less than certainty, more than bare speculation — at which legal consequences attach.
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Common Language
Modern common usage (Wiktionary): Having the faculty of reason; rational. Just, fair. Not excessive or immoderate; within due limits. Not expensive; fairly priced.
Historical common usage (Webster's 1913): Governed by reason; agreeable to reason; just; rational. Also used to describe persons endued with the faculty of reason itself.
The gap between common and legal usage is not one of opposition but of precision. Ordinary speech uses "reasonable" loosely as a synonym for sensible or moderate. Legal usage deploys it as a technical term of art that triggers specific doctrinal consequences — objective conduct tests, implied contractual terms, constitutional thresholds. When a court asks whether a fee was "reasonable," it is not asking whether the fee seemed sensible in conversation; it is asking whether the fee survives a structured legal inquiry. The common meaning misleads researchers into treating "reasonable" as a soft, impressionistic qualifier when it is, doctrinally, a load-bearing term.
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Common Confusion
REASONABLE vs. RATIONAL: In constitutional law, "rational basis" review asks whether a law is rationally related to a legitimate government interest — a highly deferential test. "Reasonable" in the Fourth Amendment context (reasonable searches and seizures) carries a different, more demanding inquiry calibrated to privacy expectations and police conduct. The two terms are not interchangeable even though they share common-language overlap.
REASONABLE vs. PROBABLE: "Reasonable suspicion" and "probable cause" are distinct constitutional thresholds with different evidentiary weights. "Reasonable and probable cause" — a phrase used in the historical sources and in older false imprisonment doctrine — should not be read as a single unified concept equivalent to modern probable cause alone.
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Core Elements
Because "reasonable" functions differently by context, its operative elements vary. In its most frequently litigated form — the reasonable person standard in negligence — the inquiry involves:
1. THE OBJECTIVE STANDARD. The question is not what the defendant actually believed or intended, but what a hypothetical person of ordinary prudence would have done in the same situation.
2. SAME CIRCUMSTANCES. The reasonable person is placed in the defendant's circumstances — not a generic vacuum — meaning physical conditions, urgency, available information, and emergency situations are factored in.
3. NOT PERFECTION. The standard does not demand optimal conduct or expert-level judgment from ordinary actors. It demands the care and skill that an ordinary member of the community would exercise.
4. CONTEXT-SENSITIVITY. The content of "reasonable" adjusts when the actor has special skills (a doctor is held to the standard of a reasonable physician), when the actor is a child, or when a statute specifies a different standard.
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Why It Matters in Research
"Reasonable" is arguably the most consequential single word in Anglo-American law. Its ubiquity is also its research trap: the same word means materially different things in a negligence case, a contract dispute, a Fourth Amendment suppression hearing, a patent prosecution, a fee petition, and an administrative challenge. Researchers must resist treating "reasonable" as a free-floating modifier and instead anchor it to its doctrinal home.
HISTORICAL CORPUS TRAP: Older sources — including all five dictionaries in this entry's source set — define "reasonable" at a high level of abstraction (agreeable to reason; just; proper) without distinguishing its context-specific applications. A researcher using Bouvier or Burrill to understand "reasonable care" in a modern negligence context will find a definition that technically fits but misses the objective-person framework that dominates contemporary doctrine.
EVOLUTION OF THE REASONABLE PERSON: The historical sources reference "reasonable man," a formulation that dominated Anglo-American law for centuries. Modern courts and statutes in most jurisdictions have replaced "reasonable man" with "reasonable person," reflecting both gender-neutral drafting norms and, in some academic commentary, substantive arguments about whose reasonableness the standard measures. Researchers working across historical and modern materials should note this terminological shift and not assume it is purely cosmetic.
REASONABLE TIME: In contract research, "reasonable time" as an implied term has deep roots and is treated by courts with substantial deference to commercial custom and trade usage. Burrill's note that "reasonable common imports what is usual" (citing Lord Ellenborough) reflects the persistent judicial instinct to resolve "reasonable" questions by reference to ordinary practice in the relevant community — an approach that survives in the Uniform Commercial Code.
CONSTITUTIONAL FRAGMENTATION: In Fourth Amendment research especially, "reasonable" does enormous doctrinal work across search-and-seizure, stop-and-frisk, and warrantless exception analysis. The cases define reasonableness through balancing tests rather than fixed rules. Researchers synthesizing historical dictionary definitions with modern constitutional doctrine should treat the dictionary as a starting point only.
CORPUS CONNECTIONS: The reasonable person standard (torts_3) is the single most important application for tort researchers. Fee petition work requires mastery of the lodestar framework (remedies_63), where "reasonable hours" and "reasonable rate" are each independent terms of art. Trade secret researchers need to understand that "reasonable measures" (ip_90) has a specific, affirmative-duty meaning in the context of secrecy maintenance that differs from general reasonableness analysis.
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Historical Dictionary Support
The five source dictionaries converge on a single-sentence core: reasonable means agreeable to reason, just, and proper. Burrill adds the practical gloss that "reasonable care is ordinary care" and that "reasonable common imports what is usual" — linking the legal standard to prevailing custom, which anticipates the objective-person inquiry of modern negligence law. Bouvier focuses on the enforceability of awards, noting that unreasonable awards (those offering no advantage to either party) cannot be enforced — an application-specific use.
Black's (2nd Ed.) is the most expansive of the historical sources, offering brief sub-entries on "reasonable act," "reasonable and probable cause," and "reasonable creature" — the last being a historical usage in the definition of murder (referring to a human being as a reasonable creature, distinguishing homicide from killing animals). This usage is now obsolete but appears in older common-law authorities and period treatises where researchers may encounter it without context.
The Rapalje & Lawrence entry as extracted is fragmentary and does not yield a coherent standalone definition of "reasonable" — it appears to be drawn from surrounding context rather than a direct definitional entry. Researchers should not rely on it for this term.
What the historical dictionaries collectively miss: the context-sensitivity and constitutional fragmentation of "reasonable" in modern doctrine. None of the sources anticipates the distinct evidentiary thresholds (suspicion, belief, cause, certainty) that twentieth-century constitutional law built around the term, nor do they reflect the objective reasonable-person standard as it was formalized through the development of negligence law.
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Jurisdictional Note
The reasonable person standard is substantially uniform across U.S. common law jurisdictions, though specific applications — particularly in professional negligence and products liability — vary by statute and local rule. In Fourth Amendment analysis, "reasonable" is a federal constitutional standard applied through Fourteenth Amendment incorporation, but state constitutions may provide broader protections under their own reasonableness frameworks. In contract law, "reasonable time" analysis under the UCC follows federal uniform law as enacted, but courts in different states apply the implied-term inquiry with varying levels of deference to trade custom.
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Encyclopedia Cross-Reference
torts_3: Negligence — The Reasonable Person Standard (The Law Mind Torts & Personal Injury Encyclopedia)
remedies_63: The Lodestar Method — Reasonable Hours x Reasonable Rate (The Law Mind Remedies & Equity Encyclopedia)
ip_90: Reasonable Measures to Maintain Secrecy (The Law Mind Intellectual Property Encyclopedia)
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