Definition
Latin. A fortuitous or inevitable accident; a loss-producing event occurring without human agency and beyond the reach of human foresight or prevention. The term encompasses any occurrence that, in the words of the historical sources, "may happen to any one" despite the exercise of the utmost diligence. In legal usage, a casus fortuitus operates as a defense or excuse: a party who suffers loss — or causes loss — through a truly fortuitous event is relieved of liability that would otherwise attach. The concept is closely associated with, but technically broader than, both act of God and force majeure, and it historically carried particular weight in the law of common carriers and maritime transport.
Common Language
Modern common usage (Wiktionary): A force majeure.
Historical common usage (Webster's 1913): The term has no entry in Webster's 1913 as an English word; it circulated as a recognized Latin borrowing in legal and educated discourse.
Editorial note: Wiktionary's equation of casus fortuitus with force majeure collapses a distinction that matters in legal research. Force majeure is a modern contractual concept that can be defined and expanded by the parties. Casus fortuitus is a common-law and civil-law doctrine with fixed doctrinal content — specifically, the requirement that no human effort or foresight could have prevented the loss. A researcher who treats these terms as interchangeable will miss cases and authorities that use one term but not the other, and will misread historical sources that deploy casus fortuitus to set a more demanding standard than modern force majeure clauses typically require.
Common Confusion
Three terms are frequently used as synonyms in both historical and modern sources but carry distinct doctrinal content:
Casus fortuitus is the broadest Latin formulation, covering any event beyond human agency or anticipation that produces a loss.
Act of God (vis divina) is a subset — natural forces exclusively, with no human element whatsoever.
Force majeure, in its modern contractual sense, is a negotiated term of art that may include events (strikes, governmental action, market disruptions) that would not qualify as casus fortuitus under classical doctrine.
Historical sources in the Law Mind corpus routinely treat these as equivalent. Modern sources, particularly in contract and insurance law, distinguish them. A researcher must attend to which term the source is actually using and in what period.
Why It Matters in Research
The primary research trap is anachronism. Nineteenth-century cases and treatises — including Kent's Commentaries, which all three source dictionaries cite — use casus fortuitus as the governing standard in carrier and bailment cases. Those sources do not use force majeure in this context. A researcher searching only for "force majeure" in historical corpus materials will miss the controlling authority; one searching only for "casus fortuitus" in modern materials will find little. Both searches are necessary depending on the period of law under investigation.
A second trap is jurisdictional layering. Civil-law jurisdictions (Louisiana, Quebec, mixed systems) absorbed casus fortuitus directly from Roman law and treat it with more structural precision than common-law courts, which tended to absorb it through the act-of-God doctrine in carrier cases. A Louisiana source using casus fortuitus may be applying a different analytical framework than a New York source using the same words.
Within the Law Mind corpus, this term connects outward to the literature on bailment, common carrier duties, marine insurance, and contract excuse. Researchers working on impossibility of performance or commercial impracticability will find casus fortuitus appearing in foundational treatise discussions that precede the modern Uniform Commercial Code vocabulary. The maxims appended to the term in the historical dictionaries — particularly Casus fortuitus non est sperandus, et nemo tenetur devinare (a fortuitous event is not to be expected, and no one is bound to foresee it) — appear as direct quotations in nineteenth-century judicial opinions and reward targeted searching.
Historical Dictionary Support
Black's, Rapalje & Lawrence, and Burrill's agree on the core: inevitable accident, loss despite all human effort, no liability for the party against whom the event occurs. All three anchor the definition to the same passage in Kent's Commentaries (volume 3, pages 216–217 and 300), which is the American locus classicus for the doctrine in the common law of carriers.
The dictionaries diverge slightly in framing. Burrill's offers the most doctrinally precise formulation, grounding the term in the Roman-law concept of damnum fatale — a loss caused by fate, which could befall any person regardless of diligence. This framing signals the civil-law lineage more explicitly than Black's or Rapalje & Lawrence, making Burrill's the most useful starting point for researching the doctrine in mixed or civil-law jurisdictions.
Rapalje & Lawrence is the most practically oriented, linking the term directly to carrier liability and cross-referencing "Act of God" and "Accident" — useful signposts for corpus navigation. Black's appends both maxims in full, which aids in locating judicial quotation.
What the historical dictionaries do not address: the relationship between casus fortuitus and contractual allocation of risk, the question of how the doctrine interacts with negligence (does any contributory fault defeat the defense entirely?), and the modern displacement of the term by statutory and contractual force majeure language. Researchers working in post-1900 materials will need to move beyond these sources.
Jurisdictional Note
Civil-law jurisdictions apply casus fortuitus with greater formal precision, often requiring proof of both externality (the event originated outside the obligor's sphere) and irresistibility (no precaution could have averted it). Common-law jurisdictions absorbed the doctrine primarily through the act-of-God defense in carrier cases and have been less systematic about its internal requirements. This divergence affects how historical sources from different jurisdictions use the same Latin term.
Encyclopedia Cross-Reference
Law Mind Encyclopedia — Force Majeure
Law Mind Encyclopedia — Act of God
Law Mind Encyclopedia — Carrier Liability