Definition
In civil law procedure, the sacramentum decisionis — also called the decisive oath or voluntary oath — is a procedural mechanism by which a party who cannot prove his case offers to submit the entire decision of the dispute to the sworn oath of his adversary. The adversary, upon receiving this offer, must either accept it (swearing to the facts as alleged against the offeror) or, if he declines to swear, tender the oath back to the original party. If the adversary refuses both to swear and to return the oath, the matter is treated as confessed against him. The mechanism thus substitutes sworn assertion for evidentiary proof when proof is otherwise unavailable, allowing a deadlocked case to reach resolution through the conscience of one of the parties.
The mechanism was not merely an evidentiary device but a terminal one: once the decisive oath was taken or tendered back and taken, the case was resolved. No further proof could be introduced on the decided question.
---
Common Confusion
The sacramentum decisionis should not be confused with the suppletory oath (juramentum suppletorium), a related but distinct civil law device. The suppletory oath is administered by the judge to a party when partial proof has been offered — it supplements insufficient evidence and remains under judicial control. The decisive oath, by contrast, is entirely party-driven: it is offered voluntarily by one party to the other and removes the judge from the decision of fact. The distinction matters in research because historical sources sometimes discuss both under the general heading of "oath evidence" without clearly separating them.
---
Why It Matters in Research
This term belongs to the vocabulary of Roman and civil law procedure, not common law. Researchers will encounter it almost exclusively in three contexts: (1) civilian jurisdiction materials and sources drawing on Roman or canon law procedure; (2) comparative law discussions contrasting civil law oath mechanisms with common law oath practice; and (3) historical English commentary that references civil law devices by way of contrast or explanation of ecclesiastical court procedure.
The primary research trap is anachronism. The sacramentum decisionis had no operative place in ordinary English common law courts, so finding the term in English-language sources nearly always signals one of two things: the author is discussing ecclesiastical or admiralty jurisdiction, which borrowed civilian procedure, or the author is making a comparative or explanatory point about civil law generally. Researchers should not assume the device was available in the forum under discussion simply because an English treatise mentions it.
The Blackstone citation in Black's (3 Bl. Comm. 342) is a signpost worth following directly. Blackstone's treatment is descriptive and comparative, not a statement of available English practice, and this distinction is easy to miss when the passage is excerpted in a dictionary entry.
Burrill's entry is incomplete in the surviving source material used here, which itself is a research signal: Burrill's Law Dictionary often provides more extensive Latin law entries than Black's or Bouvier's, and a researcher relying solely on the truncated Burrill entry would miss potential depth available in the full text.
In canon law research, watch for overlap with the juramentum in litem, another oath-based resolution mechanism. Medieval ecclesiastical procedure made extensive use of compurgation and related oath devices, and the sacramentum decisionis sits conceptually adjacent to that tradition even where the terminology differs.
---
Historical Dictionary Support
The four source dictionaries present a consistent and largely uniform account of this term, with no meaningful substantive disagreement. Black's and Bouvier's are nearly identical in phrasing, both deriving from the same civilian sources and both citing Blackstone's Commentaries. Rapalje & Lawrence tracks the same definition before the entry breaks off, apparently due to a printing or compilation artifact that runs the sacramentum decisionis entry into the immediately following entry for sacrilege — a notable editorial flaw in that source that researchers should be aware of.
Burrill's entry, as preserved here, offers only the language marker "Lat." without substantive content, making it unusable as an independent source for this term in this form.
None of the historical dictionaries engage with the procedural mechanics in depth — specifically, what happened when the adversary neither accepted nor returned the oath, or how the device interacted with appellate review. These are questions the dictionary sources leave unanswered, and researchers needing procedural detail must go directly to civilian treatises or Roman law sources.
---
Jurisdictional Note
The sacramentum decisionis was a device of Roman civil law and was carried forward in continental civilian and canon law traditions. It had no native application in English common law courts. Its relevance in English legal history is confined to ecclesiastical courts, admiralty practice, and scholarly comparative discussion. Modern civil law systems have largely replaced party-oath mechanisms with adversarial evidentiary procedures, though vestiges of judicial oath administration survive in some civilian codes.
---