Definition
Latin: "for whose good" or "for whose use or benefit." In legal contexts, cui bono functions as a principle of interpretive weight — the proposition that agreements, grants, and legal instruments should be understood by reference to whose benefit they serve. Parker, C.J. stated in 10 Mod. 135 that "cui bono is ever of great weight in all agreements." The phrase also appears in a secondary sense: for what useful purpose, or to what practical end. In this secondary meaning, it asks whether a given act, instrument, or construction serves any legitimate or useful object at all.
Common Language
Modern common usage (Wiktionary): The principle that the ultimate initiator of an action is likely the person who stands to gain from it — used rhetorically to identify motive or assign responsibility.
Historical common usage (Webster's 1913): Literally "for whose benefit"; though commonly misapplied, it came to signify "of what use" or "to what purpose," including the ultimate object of life.
The gap matters. In ordinary modern usage, cui bono has become an accusatory or investigative heuristic — roughly equivalent to "follow the money" — used to identify who probably caused something by asking who profited from it. In law, the phrase carries interpretive rather than accusatory weight: it is a principle for construing instruments and agreements in light of whose benefit they were designed to serve, not a method of assigning blame. A researcher encountering the phrase in a legal text should not import the popular sense.
Common Confusion
Cui bono is sometimes loosely rendered in modern writing as equivalent to "motive" in a criminal or investigative sense. That popular usage is a significant drift from the legal meaning. In legal instruments and maxim literature, the phrase is a rule of construction, not an evidentiary inference about culpability. The confusion is compounded by Webster's 1913 noting the phrase was already being "incorrectly understood" even in the nineteenth century.
Why It Matters in Research
Researchers will encounter cui bono in at least three distinct registers across the Law Mind corpus, and conflating them produces interpretive errors.
First, as a maxim governing construction of agreements and grants. The principle that benefit reveals intent is invoked in contract interpretation, deed construction, and the reading of ambiguous instruments. When a court or treatise writer invokes cui bono in this sense, they are applying a canon — look to who benefits to determine what the instrument means or what was intended to be conveyed.
Second, as a companion to the maxim cuicunque aliquis quid concedit. Both Black's editions follow the cui bono entry immediately with the related principle that whoever grants something is presumed to grant whatever is necessary for that thing to have effect (11 Coke, 52). These two principles appear together because they share the same underlying logic: the purpose and beneficiary of a grant inform its scope. Researchers working on implied grants, easements, or accessory rights should read them in tandem.
Third, in historical equity and common law pleading contexts, cui bono language surfaces in discussions of standing, use doctrine, and beneficial interest — areas where the identity of the true beneficiary had procedural and substantive consequences. The phrase in these contexts is doing heavier technical lifting than its surface meaning suggests.
The popular forensic meaning — identifying a wrongdoer by identifying a beneficiary — is largely absent from the classical legal sources, though it occasionally appears in older treatises on evidence and legal reasoning as a loose inference tool. Researchers should be alert to which register is in play.
Historical Dictionary Support
The historical sources are in close agreement on the core definition: "for whose good; for whose use or benefit." Black's (both editions), Burrill, and Rapalje & Lawrence all anchor the phrase to the same authority — Parker, C.J. in 10 Mod. 135 — and all quote the same formulation: "Cui bono is ever of great weight in all agreements." The convergence is significant; this is one of those maxims whose legal meaning was stable and well-settled across the nineteenth century.
Burrill is the most spare, offering the definition without elaboration. Rapalje & Lawrence add the related note that "whose it is to give, his it is" — connecting cui bono to questions of authority and ownership as well as benefit. Both Black's editions extend the entry to the companion maxim from 11 Coke, 52, treating cui bono as the head of a cluster of related constructional principles rather than an isolated phrase.
What the historical sources do not address is the forensic or investigative sense that would become dominant in popular usage. None of the nineteenth-century legal dictionaries engage with cui bono as a method of identifying wrongdoers. Webster's 1913 already flagged that the phrase was being "incorrectly understood" in common usage, and the legal dictionaries appear to have simply disregarded that development. This silence is itself informative: the legal tradition treated the phrase as belonging to construction and interpretation, full stop.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Rules of Construction; Maxims of Law; Beneficial Interest