Whereas from a professional point of view, a medical treatment might seem necessary, this treatment might be refused by the patient. In such cases, the patient's ability to determine his will is the key. Any substitutive medical decision is justified by a specific normative framework. The dialectic between a patient's inability to determine his will and the conviction of physicians about the necessity of a medical intervention is reformulated from a medical-anthropological point of view. This dialectic leads to a substitutive decision-making on the basis of a presumed will beyond the elemental will due to the breakdown of mental capacity/legal competence. After bracketing out legal concepts, assumptions mirroring canonical conceptions of individual autonomy are carved out. Then the medical encounter involving medically necessary treatments rejected by the patients is analyzed from an anthropological point of view. Hereby, both i) the asymmetry in the understanding and implementation of necessary medical measures, and ii) the emotional resonance capability between physician and patient are discussed. Based on these observations, a criticism of a solely individualistic conception of the mentally ill without any consideration of social interconnections will be developed. Finally, criteria for the assessment of mental capacity as well as for a pragmatic and prudent decision-making under uncertainty are proposed. Mental capacity and legal competence as normative concepts mean the autonomy to act towards sustainable self-care. It can be understood as the competence of judging one's own intentions; this competence includes the balanced interpretation of contingencies and forecasting the scope of consequences. The canonical dialectic between self-sufficiency (corresponding to mental capacity) and subsidiarity (according to substitutive decisions) is based, within the framework proposed here, on treatment asymmetry and personal resonance. When considering substitutive decisions, the relevant social micro systems the patient is embedded in should be taken into account. The anthropological examination of the dialectic between autonomy (based on mental capacity) and substitutive decisions leads to a reflection on individual decision-making with uncertain denouement rather than unequivocal, categorical solutions. All stakeholders and relevant others should be taken into account in the decision-making process.