Definition
In Roman and civil law, res privatae (singular: res privata) refers to things that belong to private individuals — property held by one or more persons in their own right, as distinguished from property held by the public, the state, or no one at all. The category encompasses the ordinary objects of private ownership: land, goods, money, and other assets that law recognizes as belonging to identifiable individuals or groups of individuals.
The concept is best understood as one segment of the classical Roman tripartite classification of things (res) according to their ownership status:
1. Res privatae — things belonging to private persons.
2. Res publicae — things belonging to the people or the state (seas, navigable rivers, highways, harbors, riverbanks).
3. Res nullius — things belonging to no one, capable of being acquired by the first taker.
Some civilian taxonomies also recognized res communes (things common to all, such as air and running water) and res divini juris (things under divine law, such as sacred sites and burial grounds) as additional categories outside private ownership entirely.
The Latin maxim associated with the concept — res propria est quae communis non est ("a thing is private which is not common") — functions as a residual rule: private ownership is the default legal status; everything that does not fall within a recognized public or common category is presumed to be susceptible to private ownership.
Common Confusion
Res privatae is sometimes conflated with res propria, which more specifically denotes a thing owned outright by an individual (one's own property), and with res singulorum, a near-synonymous civilian phrase meaning things belonging to particular persons. The distinctions are largely taxonomic rather than operative, but researchers reading civilian treatises or early common-law texts should treat the phrases as functionally equivalent while remaining alert to context. Res privatae should also be distinguished sharply from res publicae — its paired opposite — and from res communes, which belongs to all persons collectively rather than to the public as a legal entity.
Why It Matters in Research
Res privatae is a classificatory term from Roman civil law, not an operative doctrine of modern common law. Researchers are unlikely to encounter it in American or English case law as a freestanding legal test or cause of action. Its significance is structural: understanding where the Romans drew the line between private and public things illuminates the foundations of property theory that runs through civilian jurisdictions and through early common-law property scholarship.
Several research traps apply:
First, the term appears almost exclusively in civil law treatises and in common-law works that borrowed civilian frameworks — Bracton, Fleta, and later institutional writers. When encountered in those sources, it signals a taxonomic classification, not a rule with immediate procedural consequence.
Second, the pairing of res privatae with res publicae is diagnostically important. Wherever you find one discussed, the other is nearly always nearby. Researchers working on questions of public trust doctrine, navigable waters, highway law, or government property will frequently find res publicae — and therefore its contrast with res privatae — in the background literature even when not expressly cited.
Third, the maxim res propria est quae communis non est (quoted in Black's from an 1841 New York chancery opinion, 8 Paige 261) demonstrates that American courts occasionally reached into civilian vocabulary when confronting novel property questions in the nineteenth century. Corpus researchers searching for these concepts in American sources should search both the Latin phrases and their English translations, as courts varied in how explicitly they acknowledged the civilian lineage.
Fourth, the term has no direct statutory history in American law. Its presence in American legal writing is almost always mediated through treatise writers — Mackeldey, Justinian's Institutes, the Digest — rather than through indigenous common-law development.
Historical Dictionary Support
Black's and Burrill's converge almost entirely on substance, defining res privatae as things belonging to individuals and contrasting them with res publicae. Both cite to Justinian's Digest (1.8.1) as the primary source, with Burrill adding the Institutes (2.1.2, 4), Bracton, Fleta, and Taylor's Civil Law as supporting authorities — a richer apparatus consistent with Burrill's generally more exhaustive citation practice.
Black's contribution is the inclusion of the maxim res propria est quae communis non est with its American chancery citation, which is practically useful: it confirms the concept reached American courts and can be located in nineteenth-century equity reports.
Neither dictionary engages with the theoretical tensions within the Roman taxonomy — particularly the ambiguous boundary between res communes and res publicae, which civilian scholars have debated at length. Researchers needing that depth should consult Mackeldey's Roman Law directly, or Justinian's Institutes Book II, Title 1, where the categories are set out in the primary source.
Jurisdictional Note
Res privatae as a named legal category has no operative role in modern American common law. It retains practical significance in jurisdictions with civil law foundations — Louisiana, Quebec, and many European and Latin American systems — where the civilian taxonomy of things continues to structure property doctrine. In common-law jurisdictions, the underlying concept (private ownership as the residual category) is assumed rather than labeled.
Encyclopedia Cross-Reference
Negligence — Res Ipsa Loquitur (The Law Mind Torts & Personal Injury Encyclopedia): tangential; shares Latin "res" vocabulary but is doctrinally unrelated. No Law Mind Encyclopedia entry maps directly to the Roman property classification. Researchers may find background context useful in property-related entries.