Definition
A priori (Latin: "from what comes before") is a term borrowed from logic and philosophy, used in legal argument and judicial reasoning to describe a conclusion reached by deduction from a general principle, assumed truth, or self-evident premise — without resort to evidence, experiment, or empirical observation. In law, an a priori argument proceeds from an accepted or stipulated premise downward to a specific conclusion, rather than building upward from facts. It is the opposite of a posteriori reasoning, which derives conclusions from observed evidence and experience.
The term appears most often in appellate briefs, judicial opinions, and legal scholarship to characterize a mode of argument: the speaker is signaling that the conclusion follows as a matter of logical necessity from premises already granted, not from anything the record proves.
Common Language
Modern common usage (Wiktionary): Self-evident, intuitively obvious; presumed without analysis; based on hypothesis and theory rather than empirical evidence.
Historical common usage (Webster's 1913): Reasoning that deduces consequences from definitions or assumed principles, inferring effects from causes previously known; deductive or deductively; the reverse of a posteriori; applied to knowledge presupposed as prior to experience in order to make experience rational or possible.
The common meaning and the legal meaning are closely aligned, but the gap worth noting is one of precision. In everyday usage, a priori often carries a loose sense of "assumed without thinking it through" or even "presumed arbitrarily" — a slightly dismissive connotation. In legal and logical usage, the term is neutral or even favorable: an a priori argument is a rigorous deductive argument from admitted premises. A lawyer calling an opponent's position "merely a priori" is making a different move than a philosopher using the same phrase. Researchers encountering the term in judicial opinions should read it in the formal logical sense, not the colloquial dismissive one.
Common Confusion
A priori is frequently contrasted with — and sometimes confused with — a posteriori. The distinction matters in legal research: an a posteriori argument rests on evidence drawn from the record, from history, or from empirical study. An a priori argument rests on accepted principles or logical necessity independent of such evidence. Courts sometimes use a priori loosely to mean "from first principles" or "at the outset," which can blur the logical precision of the term. Researchers should also distinguish a priori reasoning from mere assumption or conclusory assertion: a true a priori argument is structured deductively from a stated premise, not simply an unsupported claim.
Why It Matters in Research
A priori appears across Law Mind corpus materials primarily as a term of logical description in judicial opinions, treatises, and appellate advocacy — not as a defined legal term of art with doctrinal content. Its significance for researchers is methodological rather than substantive.
First, frequency and register: the term appears with greater density in 19th and early 20th century legal materials, when legal argument was more openly philosophical and courts were more comfortable invoking formal logic. Modern judicial opinions use it more sparingly and sometimes imprecisely. When reading historical sources, expect the term to carry full logical weight; in contemporary materials, treat it as potentially rhetorical shorthand.
Second, argumentative function: encountering a priori in a brief or opinion is a signal to ask what premise is being treated as self-evident or conceded. The strength of the argument depends entirely on whether that premise is actually established. Researchers evaluating an argument described as a priori should identify the foundational premise and examine whether it was genuinely admitted or is itself in dispute.
Third, corpus connections: the term bridges legal reasoning and legal philosophy materials. It appears in discussions of natural law (where principles of justice are treated as a priori knowable), constitutional interpretation (originalism debates sometimes invoke a priori claims about original meaning), and evidence scholarship (distinguishing what a fact-finder deduces logically from what requires proof). Researchers following threads involving these topics will encounter a priori across multiple genres of legal writing.
Historical Dictionary Support
Both Black's editions offer identical substantive content, with the second edition's entry containing a minor typographical corruption ("ove which" and an ampersand for "and") reflecting the digitization of the print source — the definitions are the same. Both frame a priori as a term of logic, not a term of law, and both capture the deductive structure accurately: a general principle or admitted truth functions as cause; the conclusion follows as necessary effect.
What the historical dictionaries do not do is tell the researcher how the term functions argumentatively in legal contexts, when courts accept or reject a priori arguments, or how the term relates to specific doctrinal areas. For those purposes, the dictionary entries are a starting point only. Notably, neither edition addresses the contrast with a posteriori, which is the most useful conceptual pairing for legal researchers. Webster's 1913 provides the sharper philosophical treatment, including the Kantian dimension of knowledge prior to experience — a nuance that surfaces in natural law scholarship but rarely in mainstream doctrinal writing.