Definition
A Latin evidentiary maxim meaning "at the time of proving" or "at the moment of proof." The phrase appears in discussions of evidentiary sufficiency and the timing of when proof must be complete or operative. In practice, the maxim addresses the principle that proof must be available and effective at the point in a proceeding when it is required to sustain a claim, defense, or legal position — not merely anticipated or promised for a later stage.
The phrase is closely associated with the broader Latin evidentiary tradition and appears as a cross-reference point in discussions of *facta probantia* (facts used as evidence) and *facta probanda* (facts to be proved), the two poles of evidentiary analysis in classical common law procedure.
Why It Matters in Research
This term is a navigational challenge in historical legal research for two reasons. First, Rapalje & Lawrence does not provide a standalone definition — it redirects the reader to FACT § 4, indicating that *tum probans* functions as a term of art within a larger evidentiary taxonomy rather than as a self-contained concept. Researchers working from index references or headnotes will need to locate the parent entry to reconstruct the full analytical framework.
Second, the phrase appears primarily in evidentiary treatises and older procedural texts rather than in modern case law reporters, making it largely invisible to keyword searches in contemporary legal databases. Researchers investigating historical standards of proof, the sequence of evidentiary obligations at common law, or the procedural structure of older English and American civil proceedings are most likely to encounter this term. It belongs to a cluster of Latin evidentiary phrases — *facta probanda*, *facta probantia*, *probatio plena*, *probatio semi-plena* — that structured evidentiary thinking before the modern Federal Rules of Evidence and their state equivalents replaced Latin procedural taxonomy with codified standards.
When working in the Law Mind corpus, treat *tum probans* as a term that will not appear in isolation. Search for it in the context of the FACT entry and in treatises addressing the distinction between facts in issue and facts relevant to facts in issue.
Historical Dictionary Support
Rapalje & Lawrence cross-references *tum probans* to FACT § 4 rather than defining it independently, which is itself informative: the editors treated it as a descriptive label within an evidentiary classification system rather than as a term requiring its own doctrinal elaboration. This is consistent with the way classical Latin evidentiary phrases were used by common law treatise writers — as organizing shorthand rather than as terms with independently litigated definitions.
The maxim *Facultas probationum non est angustanda* (4 Inst. 279) — "the faculty of proofs is not to be narrowed" — appears in the same section of Rapalje & Lawrence and provides useful context. Coke's formulation reflects a common law disposition toward liberality in admitting proof, and *tum probans* fits within that tradition by focusing attention on the moment and adequacy of proof rather than restricting what may count as proof. The two phrases work together: one (Coke's maxim) addresses the breadth of admissible evidence; the other (*tum probans*) addresses the operative timing of proof.
No divergence among historical dictionary sources can be assessed here, as only Rapalje & Lawrence has been located treating this term directly.
Jurisdictional Note
As a Latin maxim from the common law evidentiary tradition, *tum probans* has no jurisdiction-specific statutory definition. Its application was uniform in the sense that it described a structural feature of proof common to English and American common law courts, but its practical relevance diminished as codified evidence rules displaced Latin procedural taxonomy in the nineteenth and twentieth centuries.