The capacity to perform acts in law, understood as the ability to express effective declarations of will, was shaped in Roman law somewhat differently than in modern legal systems because the full capacity to act in legal transactions was also determined by elements different from those recognised today. Roman law also lacks a fixed definition of capacity to perform acts in law, so a discourse about the legal standing of furiosi requires a brief introduction. As far as the furiosi are concerned, it is not possible to say how common mental disabilities were in the Roman world. In Roman law, furiosi were considered incapable of expressing their will on their own: furiosi nulla voluntas est, ‘a lunatic has no will’. The analysis of texts concerning a mentally challenged person as a wrongdoer leads unambiguously to the conclusion that a person affected by a mental disease was not held liable under the law and was not subject to punishment.