Definition
Latin maxim meaning "to the extent of his power or ability." The phrase describes an obligation or undertaking performed or to be performed according to the full measure of the actor's capacity — no more and no less. It appears most frequently in medieval legal instruments and pleadings where a party bound themselves to act within, or up to the limit of, their actual power. The companion phrase pro posse nostro carries the same meaning in the first-person plural: "to the extent of our power."
Why It Matters in Research
Researchers will encounter this phrase almost exclusively in pre-modern sources — medieval charters, Bracton's De Legibus et Consuetudinibus Angliae, and similar instruments transcribed or cited in historical collections. Its significance is not doctrinal in the modern sense; it does not define a cause of action or element of proof. Rather, it functions as a qualifying formula: the grantor, obligor, or sovereign undertook to act within the bounds of actual capacity, which served as both a commitment and a practical limitation on liability.
When tracing the phrase in the Law Mind corpus, look first in sources discussing feudal tenure obligations, charter language, and medieval warranty clauses. Bracton, folio 109, is the anchor citation in all three historical dictionaries. The pro posse nostro variant (appearing in the Carta Confirmationis of 49 Henry III, referenced in Burrill) represents the royal or collective form and may surface in searches related to the Provisions of Westminster or contemporaneous confirmatory grants.
Because the phrase operates as a qualifier rather than a substantive term, it will rarely appear as a subject heading in indices. Researchers should search body text and charter transcriptions rather than relying on index entries. The phrase is more likely to appear in Latin or mixed-language sources than in English-language pleading, narrowing the relevant corpus to medieval and early modern materials.
Historical Dictionary Support
All three historical sources — Black's (1st and 2nd editions) and Burrill's — are in complete agreement: the phrase means "to the extent of his power or ability" and each anchors the definition to Bracton, folio 109. The uniformity across sources is notable but should not be over-read; the dictionaries are essentially reprinting the same authority in succession with no independent doctrinal elaboration.
Burrill adds modest value by recording the plural form, pro posse nostro, and linking it to Cart. Conf. 49 Hen. III — a reference to a confirmatory charter from the reign of Henry III. This is the only entry of the three that situates the phrase in an actual historical instrument rather than citing Bracton alone. Neither Black's edition attempts this additional sourcing, which means Burrill is the more useful starting point for researchers who need to trace the phrase into primary charter materials.
No historical dictionary entry goes beyond the bare translation. None explains the legal context in which the formula appeared, the operative effect it was intended to carry, or its relationship to broader Bractonian conceptions of capacity and obligation. That gap is the researcher's challenge: the dictionaries confirm what the words mean but leave almost entirely silent why and where they were used.