Definition
A Latin adjective used in old English law meaning clear, lucid, or rational in mind. The term appears most often in the phrase dum dilucidus gauderet intervallis — "while he enjoyed lucid intervals" — referring to periods of mental clarity experienced by a person otherwise of unsound mind. In legal contexts, dilucidus describes the cognitive state required for a person to perform a legally valid act, such as executing a will or entering a contract, when that person suffers from a condition that intermittently impairs reason.
Common Language
Modern common usage (Wiktionary): No standard English entry. The term is not in general modern usage.
Historical common usage (Webster's 1913): Not listed as an English word. Derives from the Latin dilucidus, meaning clear, plain, or distinct — used in classical Latin to describe clarity of expression or daylight.
The Latin root carried a general sense of brightness or transparency. In law, this shifted to a specific technical meaning: not mere clarity of thought in the abstract, but the legally operative condition of a person whose mental capacity is ordinarily impaired. The legal term asks not whether someone is generally lucid, but whether a lucid interval existed at the precise moment of a legally significant act.
Common Confusion
Dilucidus should not be conflated with a general finding of competency or sanity. A person adjudged a lunatic or found of unsound mind could still act validly during a dilucidus interval. The doctrine is a narrow exception, not an alternative route to a full competency determination. Researchers conflating the two may misread historical pleadings and records that treat the same individual as both legally incapacitated and capable of binding action at different moments.
Why It Matters in Research
This term surfaces almost exclusively in historical English legal materials dealing with mental incapacity, testamentary capacity, and contract validity. Researchers working in medieval and early modern common law sources — particularly Bracton and Fleta — will encounter dilucidus in the context of property conveyances, wills, and marriage where the mental state of a party was disputed.
The practical trap: modern legal databases may not index Latin terms consistently. A search for "lucid interval" in English-language sources will often retrieve what older Latin-text sources express as dilucidus intervallis. Researchers working across both Latin manuscript tradition and later English-language digest material need to hold both formulations simultaneously.
The doctrine embedded in this term — that valid legal acts can occur during lucid intervals even for persons of generally impaired capacity — has a long common law life that extends well past the period when dilucidus itself was in active use. By the nineteenth century, English and American courts discussed lucid intervals in English without reference to the Latin term, but the underlying legal standard descends directly from the Bracton-Fleta formulation. A researcher tracing the history of testamentary capacity doctrine needs to follow this thread from dilucidus through to the later English common language cases.
Burrill's placement of this entry adjacent to the French term Dimaigne (Sunday) is an artifact of alphabetical arrangement and carries no substantive connection. Do not infer a doctrinal link.
Historical Dictionary Support
Burrill's Law Dictionary provides the sole historical dictionary entry for this term and is brief. It correctly identifies the Latin source, the English law context, and the operative phrase dum dilucidus gauderet intervallis, with citations to Bracton (fol. 43) and Fleta (lib. 3, c. 3, § 10). Both citations are genuine: Bracton's De Legibus et Consuetudinibus Angliae and Fleta are foundational thirteenth-century English legal texts. No other historical legal dictionary in the standard reference shelf covers dilucidus as a standalone entry.
What Burrill does not address: the downstream doctrinal history of lucid intervals in English equity and probate practice, the evidentiary standards courts applied to establish that an interval was genuinely lucid, or any American reception of the concept. Researchers requiring that history must move from Burrill's brief entry into the primary sources and later English-language treatise literature on mental incapacity directly.
Jurisdictional Note
As a term of art, dilucidus is specific to the English common law tradition and its medieval Latin source texts. The concept of lucid intervals — and the legal validity of acts performed during them — was received into American common law but is expressed in English in American sources. Civil law jurisdictions address analogous questions through different doctrinal frameworks and terminology.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Mental Capacity; Testamentary Capacity; Lucid Interval