Definition
A Latin term with two principal legal meanings, one rooted in Roman civil law and one in early English common law procedure.
1. Civil law: The formal statement by a plaintiff of his cause of action in an action in rem — that is, a proceeding directed against property rather than a named person. The petitio was the vehicle through which the actor (plaintiff) framed and presented his claim before the tribunal.
2. Old English law: The count or declaration in a real action — a form of pleading used in proceedings to recover land. The demandant's claim was stated in a prescribed form of words that characteristically began with the word peto (I seek or I demand). This usage is closely associated with the early common law period described by Glanvill and survives largely as a historical artifact.
A third usage — petitio principii, the logical fallacy of begging the question — falls outside legal doctrine proper but appears in legal argument and older legal writing as a term of rhetorical criticism.
Common Language
Modern common usage (Wiktionary): Used almost exclusively as a shorthand for petitio principii, the logical fallacy of assuming in one's premise the conclusion one is trying to prove.
Historical common usage (Webster's 1913): Not separately defined; treated as a Latin loan phrase.
The gap matters for researchers: a modern reader encountering petitio in a logical or rhetorical context will understand it one way, while the same word in a civil law digest or a medieval English pleading record carries an entirely different, procedural meaning. Context — especially whether the source is a Roman law text, an early English real action, or a philosophical or rhetorical argument — is the key to correct interpretation.
Common Confusion
Petitio is sometimes loosely equated with petition in the modern sense — a request directed to a court or sovereign. The overlap is real but imprecise. Modern petition is a broad procedural device; petitio in its historical legal senses was a specific, formal pleading mechanism tied either to civil law actions in rem or to the count in an English real action. The two share a Latin root and a general sense of "seeking" or "demanding," but they are not interchangeable in historical sources.
Petitio principii, the logical fallacy, is often cited in legal argument but is analytically distinct from the procedural term. Researchers should not assume that an older text's use of petitio refers to the fallacy merely because that is the term's most familiar modern meaning.
Why It Matters in Research
Researchers working in Roman law materials, civil law jurisdictions, or early English legal history will encounter petitio in sources where its meaning is procedurally specific and cannot be read through a modern lens. The term straddles two distinct legal traditions — civilian and common law — and carries different weight in each.
In civil law sources, petitio aligns with what common lawyers would recognize as the declaration or complaint in a property action. Confusing it with a petition in the modern sense (a motion or application) will distort the procedural picture entirely.
In early English real actions, the count beginning with peto was a technical formula. By the time of Blackstone and the later common law treatises, real actions had largely fallen into disuse, and the term petitio appears mainly in historical and antiquarian legal writing. Researchers using Glanvill, Bracton, or Reeve's History of English Law will encounter it in this context.
For corpus researchers in Law Mind, the term is a useful marker of source vintage and legal tradition. Its presence in a document can help date and classify the source: civil law digest, early English pleading record, or post-medieval commentary. The petitio principii sense will appear in argumentative and jurisprudential texts, not in pleading records.
Historical Dictionary Support
The historical dictionaries are in close agreement on the two core legal meanings. Black's (both editions) and Burrill offer the most complete accounts, distinguishing the civil law and English common law senses explicitly and grounding each in period sources — Justinian's Digest for the civil law usage and Reeve's History of English Law for the English pleading sense. Bouvier and Rapalje & Lawrence are spare, equating petitio simply with "count or declaration" without distinguishing the civilian context.
Anderson adds the petitio principii sense, which the other dictionaries omit, signaling that by Anderson's time the term had broadened in legal writing beyond its strictly procedural origins.
Burrill's entry is the most textually rich, noting the derivation from petere (to ask or demand) and citing the Digest directly. His observation that the English usage was "obviously borrowed" from Roman law is an important historiographical note — it flags the civilian influence on early English real action procedure, a connection sometimes underappreciated in common law-focused research.
No historical dictionary adequately addresses the transition from petitio as a technical pleading term to its survival only in historical commentary. Researchers should not expect to find petitio as a live procedural term in post-medieval English sources; by the nineteenth century, even the historical dictionaries treat it as an antiquity.
Jurisdictional Note
The civil law sense of petitio remains relevant in research touching civilian jurisdictions — Louisiana, Quebec, Scotland, and mixed-system countries — where Roman law concepts of actions in rem retain analytical significance. In purely common law jurisdictions, petitio appears only in historical and comparative materials.