Definition
In law, conjecture is a surmise or supposition arising from evidence too weak or too remote to generate actual belief. It occupies the lowest position in the hierarchy of inferential reasoning — below presumption, below probability, and well below proof. A conjecture is the idea of a fact suggested by another fact, as a possible cause, concomitant, or result, but without any positive evidence establishing it. Courts use the term primarily in the negative: findings or verdicts resting on conjecture alone are legally insufficient. The standard phrase is that a jury may not base its verdict on speculation or conjecture. The term thus functions less as an affirmative evidentiary concept and more as a boundary marker — the point at which inference becomes too attenuated to support a legal conclusion.
Common Language
Modern common usage (Wiktionary): A statement or idea that is unproven but thought to be true; a supposition based on incomplete evidence; a hypothesis considered likely but not formally proven.
Historical common usage (Webster's 1913): An opinion or judgment formed on defective or presumptive evidence; probable inference; surmise; guess; suspicion.
The gap matters for researchers. In ordinary usage, conjecture can describe a reasonable working hypothesis — something a scientist or historian might defend as the best available interpretation of incomplete data. In legal usage, the word carries no such positive valence. To say that a legal conclusion rests on conjecture is to say it is legally void. The word functions as a term of disqualification, not description.
Common Confusion
Conjecture is frequently run together with speculation, surmise, and hypothesis in legal writing, and many courts use these words interchangeably as a cluster of disqualifying terms. The functional legal meaning is the same across this cluster: an inference unsupported by legally sufficient evidence. The distinctions matter more in philosophical or scientific contexts than in courtroom application.
Conjecture should also be distinguished from presumption. Burrill's treatment of conjecture as the lowest degree of presumption is historically useful but can mislead modern researchers. In contemporary evidentiary doctrine, a legal presumption is a formal procedural device with defined procedural consequences. Conjecture is not a weak presumption — it is what remains when inference has not yet reached the threshold of a legally cognizable presumption at all.
Why It Matters in Research
The primary research significance of conjecture is its role as a standard of insufficiency. When tracking judicial review of jury verdicts, directed verdict motions, or sufficiency-of-the-evidence standards, conjecture appears consistently as the negative pole — the condition that, if present, compels reversal or dismissal. Researchers examining appellate opinions on evidentiary sufficiency will encounter the term as a fixed formula rather than an analytical concept requiring fresh interpretation.
Two traps appear in historical sources. First, older treatise writers, particularly in the civil law tradition drawn on by Bouvier and Burrill, placed conjecture within a formal taxonomy of presumptions running from conjectura at the bottom through praesumptio to probatio. Modern American evidence law does not use this taxonomy operationally, so applying the historical hierarchy to contemporary doctrine produces distortion. Second, Black's 1st and 2nd editions cite Burrill's Circumstantial Evidence as the controlling authority on the definition — researchers working with that passage should treat it as a mid-19th century framing that courts absorbed into boilerplate rather than as a continuously analyzed doctrine.
For corpus researchers, conjecture connects most directly to the evidentiary sufficiency cluster: inference, presumption, circumstantial evidence, and the directed verdict line of cases. It also appears in pleading contexts, where allegations characterized as conjectural may be dismissed for failure to state a claim, though this usage is less formalized than the evidentiary one.
Historical Dictionary Support
The five source dictionaries show strong agreement on the core definition: conjecture is a slight degree of credence arising from evidence too weak or too remote to cause belief. Black's 1st and 2nd editions, Bouvier, and Rapalje & Lawrence are essentially identical in substance. Burrill adds the most structural content by situating conjecture as the lowest degree of presumption and citing Domat's Civil Law — a signal that the concept was imported from civilian jurisprudence into common law treatise writing.
The civilian heritage is visible throughout. Bouvier cites Mascardus, a 16th-century Italian proceduralist, for the proposition that conjecture produces only a slight degree of credence. Burrill's Latin cross-reference (conjectura) confirms the term's learned-law origins. What historical sources do not address, and what modern doctrine requires, is the procedural consequence of conjecture — its role in defining the floor below which evidence cannot sustain a verdict. That development belongs to 20th-century American appellate case law, not to the dictionary tradition.
Black's 2nd edition adds a case citation (Weed v. Scofield, 73 Conn. 670) to anchor the definition in American authority, which is absent from earlier editions. This modest addition reflects the shift from treatise-based to case-based legal authority that accelerated in the late 19th and early 20th centuries.
Jurisdictional Note
The term operates consistently across American common law jurisdictions as a standard of insufficiency, and no significant jurisdictional variation affects its core meaning. Researchers in civil law systems should be aware that the civilian taxonomy reflected in Bouvier and Burrill — placing conjecture formally within a hierarchy of presumptive proof — is not operative in American courts, though it informs the historical vocabulary.