The essay aims to delve into the roots of the principle ‘nullus idoneus testis in re sua intelligitur’ to verify whether the prohibition of testifying in trials of direct interest constitutes an essential element of the discipline of testimony in the Roman private trial. The analysis of the sources in question, highlighting the relativism that characterizes the notion of ‘impartiality’ of the witness, leads to excluding such an eventuality, connecting the rule to the developments of the procedural discipline known in the classical and post-classical ages.