Definition
Ultra posse is a Latin maxim meaning "beyond power" or "beyond ability." It expresses the legal and logical principle that no person, institution, or entity can be required to do what is impossible. The full form of the maxim is ultra posse non potest esse obligatio — an obligation cannot exist beyond possibility — or in its compressed form as a paired maxim: ultra posse non potest esse, et vice versa, meaning "what is beyond possibility cannot exist, and the reverse."
In legal application, ultra posse operates as a limiting principle: a duty, obligation, or legal requirement cannot bind a party to perform what is genuinely impossible. The law does not compel the impossible. This principle surfaces in contract law (impossibility of performance), in corporate law (acts beyond a corporation's legal capacity), in constitutional law (limits on governmental authority), and in general equity (courts will not decree performance of the impossible).
Common Language
Modern common usage (Wiktionary): Not a standard entry; the phrase is Latin and appears primarily in legal and philosophical contexts.
Historical common usage (Webster's 1913): Not listed as a common English term; treated as a learned Latin phrase.
Editorial note: Ultra posse has no ordinary English counterpart and functions purely as legal and philosophical Latin. Researchers should not expect to encounter it outside legal texts, treatises, or maxim collections. Its meaning is fully embedded in the Latin and requires no gap analysis against everyday usage.
Common Confusion
Ultra posse is closely related to — but distinct from — ultra vires. Both use the prefix ultra ("beyond"), and both operate as limiting principles on legal capacity. The difference is fundamental: ultra vires means beyond the scope of granted legal authority (a corporation acts outside its charter), while ultra posse means beyond the bounds of physical or legal possibility (no one can be obligated to do what cannot be done). A corporation can act ultra vires on something that is physically possible but legally unauthorized. Ultra posse, by contrast, addresses absolute impossibility — not unauthorized action, but impossible action.
Researchers finding references to "ultra" maxims in older corporate law cases should distinguish carefully which principle is being invoked.
Why It Matters in Research
Ultra posse appears infrequently as a standalone term in case law but functions as a background principle that surfaces under several doctrinal labels. Researchers should be alert to the following:
In contract and obligation cases, the principle is more often expressed as "impossibility of performance" in modern decisions, with the Latin maxim appearing in older equity opinions and treatise commentary. Historical sources will use the Latin; modern cases will use the English doctrine. These are the same principle and should be cross-searched.
In corporate law, ultra posse underlies arguments that a corporation cannot be held to a contract or obligation that its charter made impossible to fulfill — distinct from ultra vires, where the act was possible but unauthorized. The cases cited in Black's 2nd Ed. — including Central Transportation Co. v. Pullman's Palace Car Co., 139 U.S. 24 (1891) — involve corporate incapacity arguments where the two principles intersect. Researchers working in historical corporate law must hold both concepts in mind simultaneously.
The maxim also appears in early constitutional and governmental power cases where courts reason that a grant of power cannot require performance of the impossible or self-contradictory.
Because ultra posse is a maxim rather than a doctrine with its own case law taxonomy, it will rarely appear as a headnote or index term in modern legal databases. Searches should focus on the underlying doctrines (impossibility, incapacity, legal disability) or on treatise and maxim collections in historical sources.
Historical Dictionary Support
Black's Law Dictionary (2nd Ed.) presents ultra posse as a maxim with the formulation ultra posse non potest esse, et vice versa — "what is beyond possibility cannot exist, and the reverse." Black's deploys this as a foundational logical-legal principle and supports it with a cluster of cases spanning contract, corporate, and governmental contexts: Minnesota Thresher Mfg. Co. v. Langdon (corporate obligations), State v. Morris & Essex Co. (governmental power), Central Transportation Co. v. Pullman's Palace Car Co. (corporate incapacity), Latimer v. Bard (equitable obligation), and Edwards County v. Jennings (municipal obligation).
The historical dictionaries treat ultra posse primarily as a maxim of universal application rather than a doctrine specific to any single field. This framing is accurate but underweights the maxim's practical significance as a limiting principle in corporate incapacity cases, where it frequently pairs with ultra vires analysis without being clearly distinguished from it. The historical sources do not flag this conflation risk, which is the most important trap for modern researchers working in the historical corpus.
Jurisdictional Note
Ultra posse as a principle applies across common law jurisdictions without significant variation — the impossibility of demanding the impossible is treated as universal. However, the specific doctrines that implement the principle (impossibility of performance, frustration of purpose, corporate incapacity) vary in their elements and defenses by jurisdiction and should be researched separately.
Encyclopedia Cross-Reference
Corporate Formation — Corporate Purpose, Powers, and Ultra Vires Doctrine (The Law Mind Business Organizations & Corporate Law Encyclopedia)