Definition
A Latin term meaning "opinion" in the broad sense, used primarily in civil law and Roman law contexts to denote a belief, judgment, or legal conviction held by jurists, courts, or the community at large. In legal usage, *opinio* carries weight beyond mere personal belief — it refers to a recognized form of legal authority derived from the persuasive force of shared professional or public conviction.
The term appears most often in two significant compound forms:
1. *Opinio communis* (also *opinio vulgaris*): The common or generally prevailing opinion among jurists or the legal community. Under civil law doctrine, this shared professional consensus carried normative force in judgment and legal counsel, functioning as a secondary source of law where positive enactment was silent or ambiguous.
2. *Opinio juris* (short for *opinio juris sive necessitatis*): The belief that a practice is legally obligatory — the subjective element required alongside consistent state practice to establish a rule of customary international law. This is the more widely used form in modern legal scholarship and is treated separately in most contemporary dictionaries.
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Common Language
Modern common usage (Wiktionary): "Opinion" in English means a view or judgment formed about something, not necessarily founded on fact or knowledge.
Historical common usage (Webster's 1913): "A belief stronger than impression, less strong than positive knowledge; a conclusion or judgment held with confidence but falling short of certainty."
The gap between common and legal meaning is significant. In ordinary English, "opinion" implies subjectivity and personal variability — one opinion is as good as another. In the civil law tradition, *opinio communis* is the opposite: it is precisely the convergence of individual opinions into a recognized, authoritative consensus that gives the concept legal force. The term thus describes a source of law, not merely a personal view.
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Common Confusion
*Opinio* as a general term is frequently collapsed with *opinio juris*, the doctrine central to customary international law. Researchers should be careful: the historical civil law usage tracked in Burrill and similar dictionaries concerns domestic juristic consensus (*opinio communis*), not the two-element test for international custom. The two concepts share a root but operate in entirely different legal frameworks. Conflating them can produce significant analytical errors when researching either Roman law doctrine or the development of international law.
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Why It Matters in Research
The term *opinio* is a navigational crossroads in the Law Mind corpus, pointing in at least two directions simultaneously.
First, for researchers working in Roman law, civil law, or the history of the common law's reception of civilian doctrine, *opinio communis* is a live concept. Pre-nineteenth century legal literature — particularly treatises drawing on Justinian's *Corpus Juris Civilis* and the Glossators — invokes *opinio communis doctorum* (the common opinion of the doctors) as a source of binding authority. Burrill's entry reflects this tradition directly. When reading early chancery materials, equity treatises, or ecclesiastical law sources in the corpus, encountering *opinio* without a modifier usually signals this civilian usage.
Second, for researchers working in international law materials from the late nineteenth century onward, the critical compound is *opinio juris*. Its appearance accelerates dramatically after the Statute of the Permanent Court of International Justice (1920) codified customary international law as a formal source. Sources before that date may use the phrase without the technical weight it later acquired.
A practical trap: abridged digests and older encyclopedias sometimes cite *opinio communis* as if it were a self-executing rule — that majority juristic opinion automatically became law. This overstates the civilian position, which was itself contested. Burrill's entry hints at this, noting "much controversy among the" civilians on the question. Researchers should not assume uniformity in how historical sources apply the concept.
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Historical Dictionary Support
Burrill's Law Dictionary provides the primary surviving entry in the Law Mind shelf for this term. Burrill accurately identifies the core civil law rule — that *opinio communis judicando et consulendo complecti debet* (common opinion ought to be embraced in judging and in giving legal advice) — and correctly flags that this rule was contested. The entry is compressed but reliable as a point of entry into civilian doctrine.
What Burrill's treatment does not address is the subsequent development of *opinio juris* as a discrete technical term in international law, which had not yet fully crystallized as doctrine at the time of the dictionary's composition. Researchers should treat Burrill as authoritative for the historical civilian meaning while recognizing that the term's most prominent modern usage lies in international law scholarship that postdates the classical dictionaries on the Law Mind shelf.
No other source dictionary in the current corpus provides a competing or supplementary entry for this term.
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Jurisdictional Note
*Opinio communis* as a formal source of law is a civil law concept without a direct common law equivalent. Common law jurisdictions developed persuasive authority through precedent rather than juristic consensus, making this term largely absent from English and American domestic legal practice after the seventeenth century. *Opinio juris* remains relevant across all jurisdictions insofar as they engage with customary international law obligations.
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