Definition
Post hac is a Latin adverbial phrase meaning "after this" or "from this point forward." In legal writing and analysis, it signals that something occurs subsequent to a referenced point in time — an event, an act, a judgment, or a legal threshold. It functions as a temporal marker indicating prospective or subsequent effect, as opposed to retroactive or prior operation.
The term appears most frequently in two contexts:
1. Temporal sequencing in legal instruments and opinions: Post hac marks when an obligation, right, or condition takes effect going forward from a specified moment.
2. Logical analysis: Post hac is the compressed form of the logical fallacy post hac ergo propter hac — more commonly rendered as post hoc ergo propter hoc — meaning "after this, therefore because of this." This is the error of inferring causation from mere temporal sequence. A lawyer arguing that Event B was caused by Event A simply because B followed A commits this fallacy.
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Common Language
Modern common usage (Wiktionary): Not independently listed as a common English term; encountered primarily as a fragment of the logical fallacy phrase.
Historical common usage (Webster's 1913): Not entered as a standalone term; recognized as part of the scholastic phrase post hoc ergo propter hoc in logical and rhetorical usage.
Editorial note: In everyday educated speech, "post hac" rarely appears alone — the phrase is almost always encountered as part of the fallacy construction. Legal and scholarly writing sometimes abbreviates to "post hac" when the fallacy reference is already established in context. Researchers should not assume the abbreviated form always carries the fallacy connotation; in some instruments and opinions it is purely temporal.
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Common Confusion
Post hac / Post hoc: The forms "post hac" and "post hoc" are frequently treated as interchangeable, and in many sources they effectively are. Technically, the difference is grammatical: hac is the ablative feminine singular of hic, while hoc is the ablative neuter singular. Because the noun being modified controls the form, either may appear depending on the antecedent noun in the original Latin construction. For practical legal research purposes, both forms signal the same temporal meaning and the same fallacy risk. Do not conclude that one form is a typographical error or a different legal concept.
Post hac / Nunc pro tunc: These are conceptual opposites. Post hac operates prospectively — from this point forward. Nunc pro tunc operates retroactively — now for then. Courts and practitioners occasionally err in labeling a prospective order nunc pro tunc when they mean its effect begins at the present moment, not that it relates back to a prior date. The distinction is material where effective dates determine rights.
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Why It Matters in Research
The primary research trap with post hac is the fallacy context. Legal arguments frequently depend on causal chains — negligence, but-for causation in tort, proximate cause in criminal law, reliance in contract. When reviewing briefs, opinions, or secondary sources, spotting a post hac ergo propter hac structure is a signal that the causal link may be asserted but not proven. Courts often identify and reject such reasoning without naming the fallacy explicitly, so a researcher must recognize the pattern even when the Latin phrase is absent.
In historical legal sources, post hac appears as a straightforward temporal signal, not as a flagged fallacy term. Black's Law Dictionary treats it simply as "after this; after this time; hereafter." Researchers using older digests or treatises will encounter this usage in instrument drafting, order language, and opinion sequencing — the fallacy meaning is an overlay from logic and rhetoric, not from legal lexicography proper.
Jurisdictional variation does not affect the meaning of post hac as a Latin phrase, but the significance of prospective versus retroactive effect — which post hac directly implicates — varies considerably by jurisdiction and by subject matter (tax, family law, criminal sentencing). When post hac appears in an order or statute, tracing the effective date question into local rules and case law is essential.
For researchers working in appellate litigation, evidence, or expert witness contexts, the fallacy form (post hac ergo propter hac) intersects with standards for admissibility of expert causation testimony. Challenges to expert opinions on causation often rest, at bottom, on this logical problem even when framed under Daubert or Frye standards.
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Historical Dictionary Support
Black's Law Dictionary provides the sole historical legal dictionary entry and keeps it minimal: "Lat. After this; after this time; hereafter." This is accurate as far as it goes. The entry treats the phrase as a temporal marker only and makes no mention of the logical fallacy construction — a significant gap given how frequently the phrase appears in legal argument in precisely that context.
No other surveyed historical legal dictionary enters post hac independently. The fallacy phrase post hoc ergo propter hoc receives treatment in classical logic references and rhetoric manuals, which are not legal dictionaries, but which inform the argumentative context any legal researcher will encounter. The absence of the fallacy dimension from legal lexicography reflects that the phrase migrated into legal usage from scholastic logic; legal dictionaries recorded only the plain temporal meaning they needed for instrument and opinion drafting.
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