The Trojan Horse of ‘impossibility’: Unmasking epistemic boundaries and normative overreach in ISO 21043-4

This paper critically examines the conceptual and practical challenges raised by forensic conclusions that assert categorical certainty about propositions of interest. Such conclusions rest on the examiner's judgment that it is ‘impossible' for the observations in question to arise under any alternative proposition – a stance now formalised in ISO 21043-4 (Forensic Sciences – Part 4: Interpretation). Our paper provides a concise overview of the necessity and role of expert witness testimony in contemporary legal systems. It outlines the legal framework governing scientific evidence and draws attention to key elements of ISO 21043-4. Thereupon we examine the reasoning patterns underpinning ISO 21043-4, with particular attention to the vast hypotheses-space involved in relating results to a specific proposition. It introduces a key distinction between the Set of Relevant Alternatives – propositions that must be explicitly ruled out – and the Set of Silently Obviated Alternatives, which are dismissed without formal consideration. The paper then analyses the ‘Trojan Horse' effect of invoking ‘impossibility' in forensic reporting, showing that categorical conclusions rest not solely on methodological validity but on normative choices that exceed empirical warrant. These issues are situated within epistemology to highlight both the limits of justified forensic claims and the asymmetry they impose on defendants. Thereupon we highlight the gap between the generality of scientific conclusions and the singularity of legal decisions regarding unique historical events. It argues that the ISO's ‘impossibility' clause fosters scientific imperialism, allowing experts to arbitrarily reshape both epistemic and normative limits. Drawing on jurisprudential concepts, our analysis shows how this practice cedes decision-making prerogatives from legally empowered fact-finders to scientists, undermining the justice system’s legitimacy.