The contemporary medicine goes through a deep transition with regard particularly to humanization; modern ethics has effected sundry changes in attitudes to patients and one of them is the edification of her/his right to make choices about her/himself. The concept of autonomy however meets the problem of assessment of the level to which the concrete patient is capable of making decision due to various impairments and strains. One possible criterion, which is compliant with traditional philosophy and which is based on rationality is suggested here.

        Key words: autonomy, informed consent, competence, rationality, argument.