Definition
Res communes (Latin: "common things") is a classification from Roman civil law describing things that belong to no one but are available for use by all persons by nature. The defining characteristic is that such things cannot be owned — not by individuals, not by the state, and not by any corporate body — yet all persons may freely use them. Classic examples from the Roman sources are air, running water, the sea, and the seashore.
The concept is part of a broader Roman taxonomy of things (res) according to their legal status and capacity for ownership. Res communes sit at one end of that taxonomy: incapable of private ownership and open to all. This distinguishes them from things held by the state, things held in common by a defined group, and things capable of individual ownership.
Common Confusion
RES COMMUNES vs. RES PUBLICAE vs. RES NULLIUS: These three categories are frequently conflated, including in the historical sources themselves. Res publicae are things owned by the state but made available for public use (e.g., roads, harbors). Res communes are owned by no one and available to all — the state has no proprietary claim. Res nullius are things owned by no one that can be acquired through occupation (e.g., wild animals before capture). The practical stakes of the distinction: res communes cannot be privatized even by sovereign grant; res publicae can theoretically be alienated by the state; res nullius can be acquired by the first taker. Courts and commentators working outside the civil law tradition often blur these lines, particularly when discussing shoreline rights and fishery access.
Why It Matters in Research
The critical research task with res communes is distinguishing it from neighboring categories that appear in the same sources: res publicae (public things owned by the state), res universitatis (things belonging to a municipal body), and res nullius (things belonging to no one but subject to acquisition). Historical sources handle these distinctions inconsistently. Burrill explicitly flags that Bracton and Fleta distinguish between common things and public things while Justinian uses the terms interchangeably — a warning that the same Latin phrase in two different sources may carry different doctrinal weight.
Researchers working in Anglo-American common law will encounter res communes primarily in discussions of the public trust doctrine, navigable waters, tidelands, and shoreline access. American courts drawing on natural law and civil law traditions used the res communes framework — sometimes loosely — to explain why certain resources resist private appropriation. The category matters in that lineage but has never been formally codified in common law the way it was in Roman law. Be cautious about reading modern public trust doctrine arguments backward into the civil law category; they share intellectual ancestry but are not identical.
In environmental law scholarship and property theory, res communes has experienced a revival as a conceptual tool for discussing global commons — the atmosphere, the deep seabed, ocean fisheries — where no sovereign can assert title. This modern usage extends well beyond the Roman source material, and researchers should treat contemporary invocations of the term as analytical borrowings rather than strict legal doctrine.
For corpus research, res communes appears most densely in: treatises on property and civil law, natural law theory texts from the 17th and 18th centuries, admiralty and maritime law discussions of the sea and shoreline, and 19th-century American works on riparian rights and public lands.
Historical Dictionary Support
Black's Law Dictionary provides the core definition but the surviving fragment in the source material cuts off before it completes the sentence, noting only that res communes "can never be exclu-" — the implication being that such things can never be exclusively appropriated. The truncation is editorially significant: the definition's emphasis was on incapacity for exclusive ownership, which is the doctrinal center of gravity.
Burrill's Law Dictionary is the richer source here. Burrill gives the standard Institutes and Digest citations (Inst. 2.1.1; Dig. 1.8.2.1), grounds the term in Bracton and Fleta for English readers, and then does important work distinguishing how later civilians refined the category. The distinction Burrill preserves — that later civilians separated res communes (accessible to all by nature) from res publicae (owned by the state but open to public use) — is the distinction that matters most for legal research. Justinian's conflation of the two created interpretive difficulties that persisted through the medieval glossators and into English natural law writing.
Neither Black's nor Burrill fully captures how the category was absorbed and transformed in English common law, where the res communes framework influenced doctrine on the foreshore and the public right of fishing without ever being formally received as Roman law.
Jurisdictional Note
The res communes concept has no direct statutory reception in American or English law, but its influence is felt in public trust doctrine litigation, where courts in states like California, New Jersey, and Hawaii have drawn on natural law and civil law sources to limit private ownership of tidal and navigable resources. The degree of that influence varies significantly by jurisdiction and by the intellectual tradition of individual courts.