An a priori assumption is a proposition adopted before examining case-specific evidence, based on logic, policy or generalised expectations rather than proof. In legal practice it describes a mode of reasoning, not a rule; it is not defined in UK or Irish legislation or as a term of art in case law, but appears in judgments, pleadings and academic analysis.Key features and usage:- Contrasts with an a posteriori inference drawn from admissible evidence.- Has no evidential or procedural effect and is distinct from legal presumptions (of law or fact) and from judicial notice, which carry defined consequences for proof.- Appellate courts may criticise findings grounded in a priori assumptions about credibility, human behaviour, risk or market dynamics; tribunals are warned against stereotyping in discrimination, criminal and asylum contexts.- In statutory interpretation and judicial review, courts avoid a priori policy assumptions, focussing instead on text, context and purpose.Practical significance: arguments built on a priori assumptions are vulnerable to challenge as speculation. Practitioners should anchor submissions in admissible evidence and clear legal principle and, where opposing such reasoning, emphasise the burden and standard of proof.Usage is broadly consistent across England and Wales, Scotland, Northern Ireland and Ireland.