This paper proposes a novel method for identifying the public evaluative standards that contribute to the classification of certain conditions as mental disabilities. Public evaluative standards could contribute to ascertaining disabilities by outlining characteristics whose presence beyond a threshold is fundamentally important for the life of a person and whose absence or reduced occurrence constitutes a disability. Additionally, they can participate in determining disabilities by delineating particularly grave difficulties, impairments, or incapacities. Our method relies on a model of public justification of evaluative standards that is inspired by Gerald Gaus’s theory of public reason. Thus, our approach recommends the justification of evaluative standards through sound deliberative routes from reasons accessible to all persons who participate in the process of justification and the convergence of what is justified in this way to each of them. We deem that disabilities could be caused both by problems in the internal characteristics of a person as well as by unfairness or a lack of hospitality in external circumstances. This is why the method of justification is applied to the assessment of those circumstances as well. If social or environmental circumstances cannot be justified through the convergence of reasons accessible to all persons involved in the process of justification, we have reasons to exclude the presence of a disability and ascertain the presence of inadequate external conditions.