Definition
A Latin term from the civil law meaning deaf, or a deaf person. Used in legal contexts to describe a person who lacks the faculty of hearing, with direct consequences for legal capacity — particularly regarding the ability to make a will, enter contracts, or otherwise perform legal acts requiring the ability to give and receive spoken communication.
The compound phrase surdus et mutus (deaf and dumb) denotes a person who is both deaf and mute, a category that Roman law and its successors treated with particular attention, as such persons faced compounded limitations on their capacity to participate in formal legal proceedings and transactions.
Common Language
Modern common usage (Wiktionary): In general English, "surd" (the adjectival form surviving in modern usage) refers in mathematics to an irrational number or an unresolvable root expression. The root surdus also survives in linguistics to mean a voiceless consonant.
Historical common usage (Webster's 1913): Webster's 1913 records "surd" as both a mathematical term for an irrational quantity and a phonetic term for a voiceless sound, tracing both to the Latin surdus meaning deaf or mute — hence "unheard" or "without voice."
The gap between common and legal meaning is complete: the modern English reader encountering "surd" in a mathematical or phonetic context will find no guidance toward the civil law category of a person with legal incapacity due to deafness. Researchers working in historical legal sources must treat surdus as a technical term of Roman and civil law, not a cognate of its surviving English forms.
Why It Matters in Research
Surdus appears in Roman law sources bearing directly on legal capacity, and researchers tracing the history of disability and legal personhood will encounter it in the Institutes and Digest. The Institutes at 2.12.3 address the capacity to make a will, and the Digest passages cited by Burrill (28.1.6.1; 28.1.7; 50.17.124) bear on testamentary formalities and the general rules of legal capacity. English legal writers following the civil law tradition — including Fleta — carried the term forward, making it relevant to medieval and early modern common law research as well.
The critical research trap is assuming that Roman incapacity rules for the deaf translated directly into English common law. They did not do so uniformly. Roman law developed nuanced distinctions — whether deafness was congenital or acquired, whether the person could read and write — that common law courts and commentators sometimes adopted, sometimes ignored, and sometimes collapsed into simpler categorical rules. A researcher relying only on English-language sources may miss the civil law substrate entirely.
The compound surdus et mutus is the more frequently litigated category in historical sources, because a person who was both deaf and mute could neither hear the words of a formal transaction nor respond orally, eliminating participation in the oral formalities central to Roman and early common law practice. Researchers should watch for this phrase in ecclesiastical court records, probate proceedings, and chancery materials, where questions of testamentary capacity were most likely to arise.
Historical Dictionary Support
All three source dictionaries agree on the core definition: surdus means deaf or a deaf person in the civil law, with surdus et mutus denoting the deaf-mute. The agreement is near-verbatim, as all three ultimately trace to the same Roman law sources.
Burrill is the most useful of the three for research purposes, providing the fullest set of source citations — the Institutes, multiple Digest passages, and Fleta — and giving a researcher a working bibliography for primary source investigation. Black's (both editions) is briefer, providing the Institutes citation only and appending the definition of surenchère without transition, which reflects the dictionary's alphabetical compression rather than any substantive limitation.
None of the historical dictionaries addresses what happened to the surdus category as it was received and modified in English law, nor do they engage with the question of partial incapacity — for instance, a person who was deaf but literate, who might in later periods be accommodated by written formalities. Researchers should not assume the Roman rule traveled unchanged.
Jurisdictional Note
Surdus as a technical term belongs to the civil law tradition and is most directly operative in Roman law, canon law, and civil law jurisdictions. Its influence on English common law was indirect, primarily through ecclesiastical courts and equity. Researchers working in Scottish, Louisiana, or Quebec sources — jurisdictions with stronger civil law inheritance — will encounter the concept more directly than those working in purely common law English or American materials.