Definition
A posteriori (Latin: "from what comes after") is a term borrowed from logic and philosophy denoting a mode of reasoning that proceeds from observed facts or experience to a conclusion. In legal usage, an a posteriori argument takes ascertained facts as its starting point — treating them as effects — and works backward by synthesis and induction to identify their cause or establish a broader principle. It is the inferential movement from the specific and concrete to the general.
The term appears in legal writing primarily as a descriptor of argumentative method rather than as a substantive legal concept in its own right. A court or advocate reasoning a posteriori builds a conclusion by accumulating evidence, observed outcomes, or established facts and drawing an inference from them — as opposed to deducing a conclusion from an antecedent rule or principle.
Common Language
Modern common usage (Wiktionary): Involving induction of theories from facts; derived from or based on observation or experience.
Historical common usage (Webster's 1913): Characterizing reasoning that derives propositions from observation of facts, or by generalization from facts arrives at principles; inductive or empirical — the reverse of a priori reasoning.
The common and legal meanings are substantively identical here: both treat a posteriori as a descriptor for inductive, experience-based reasoning. The legal significance lies not in any gap between legal and common meaning, but in recognizing that when the term appears in judicial opinions, briefs, or legal scholarship, it is almost always functioning as a methodological label for a particular form of argument rather than as a term of art carrying independent legal weight.
Common Confusion
A posteriori is routinely paired with, and sometimes confused with, its counterpart a priori. The distinction matters for legal research:
An a priori argument proceeds from a pre-existing principle, definition, or rule downward to a conclusion — deductive, top-down reasoning. An a posteriori argument proceeds upward from observed facts to a principle — inductive, bottom-up reasoning.
In legal texts, the confusion tends to surface when editors or readers encounter both terms in close proximity in philosophical or jurisprudential writing. A source describing a court's reasoning as a posteriori is asserting that the court derived its rule from the pattern of cases before it, not from an abstract principle. Misreading it as a priori reverses the characterization entirely.
Why It Matters in Research
A posteriori is a term that rarely anchors a legal argument on its own — it is a descriptor that characterizes how an argument is constructed. Researchers encountering it in primary or secondary sources should treat it as a signal about methodology, not substance.
In historical legal texts and treatises, particularly those from the 18th and 19th centuries when natural law theory and legal philosophy were more visibly integrated into doctrinal writing, a posteriori appears with some regularity to describe empirical or common-law reasoning. Common law methodology — building doctrine case by case from accumulated decisions — is itself frequently characterized as a posteriori in jurisprudential literature. Researchers working in legal philosophy, jurisprudence, or the history of legal thought will encounter it far more often than those working in transactional or procedural materials.
The term also appears in evidence scholarship and in discussions of judicial fact-finding to distinguish between reasoning from abstract rules (a priori) and reasoning from the record (a posteriori). In that context, misidentifying the direction of reasoning can distort the analysis of how a court reached its holding.
When the term appears in a source, check whether the author is using it precisely in the philosophical sense or loosely to mean simply "based on the facts." The latter usage, while technically consistent, sometimes obscures whether the author intends a genuine logical claim about the structure of the argument or merely means "empirically supported."
Historical Dictionary Support
Both editions of Black's Law Dictionary provide virtually identical definitions, and both faithfully track the standard philosophical usage: an argument founded on experiment or observation, taking ascertained facts as an effect and proceeding by synthesis and induction to their cause. The definitions are stable across editions, which reflects the term's status as an imported logical concept rather than a term that evolved through legal practice.
Neither Black's edition situates the term within a broader discussion of legal reasoning methodology, which is a notable omission. The definitions are accurate as far as they go, but a researcher relying solely on Black's would not understand how the term functions in the jurisprudential literature — particularly its role in distinguishing common-law inductive reasoning from civil-law deductive reasoning, a distinction that was live in 19th-century Anglo-American legal theory. Webster's 1913 is actually more useful here than either Black's edition for conveying the full logical context, particularly its explicit identification of a posteriori as the reverse of a priori reasoning.