The Contemporaneity Assumption: Intergenerational Equity as an Unwritten Principle of UK Constitutional Law

UK constitutional law rests on a tacit and largely unexamined assumption: that legal obligations, rights, and constitutional principles operate between contemporaneous parties. This article calls this the contemporaneity assumption and argues that it is the root cause of a systematic failure to protect future generations from present constitutional excess. The argument proceeds in three stages. First, it shows that the assumption is not entailed by any constitutional principle — not by parliamentary sovereignty, the rule of law, or democratic accountability — but is instead an inherited default of legal reasoning never subjected to principled scrutiny. Second, it demonstrates that UK law already contains the conceptual resources to displace the assumption: the unwritten constitutional principles doctrine, the fiduciary theory of public office, and an emerging line of climate jurisprudence, including KlimaSeniorinnen and Finch, implicitly organised around temporal accountability. Third, it proposes fiduciary constitutionalism as the organising framework connecting these elements. Recognised as an unwritten constitutional principle addressed primarily to Parliament and the executive, intergenerational equity would in its ordinary judicial operation require no new task of courts — it would name what they are already doing and extend its logic to cases it has not yet reached.