<p>When a patient lacks capacity, medical decisions on their behalf are made according to an advance directive or by surrogate decision making. Often, however, patients’ previously expressed wishes are ambiguous, vague, inconsistent, or fail to anticipate the patient’s current condition. In this paper, we argue that when patient’s wishes are not clear, surrogates must utilize interpretative principles to reach a decision regarding treatment. We identify three such principles: the value-substitution, value-coherence, and volitional principles. We argue that the volitional principle is the most reliable way of capturing what the patient would have wanted when they no longer possess decisional capacity. This approach tasks the surrogate with identifying a medical choice close to what the patient would have agreed to based on previously expressed wishes without attributing the surrogate’s own values to the patient or attempting to provide an interpretation consistent with the patient’s other values. This approach is best positioned to support patients’ sovereignty for those who were previously able to express wishes for or against life-sustaining treatment.</p>