Decision making for incompetent patients is a much-discussed topic in bioethics. According to one influential decision making standard, the substituted judgment standard, the decision that ought to be made for the incompetent patient is the decision the patient would have made, had he or she been competent. Although the merits of this standard have been extensively debated, some important issues have not been sufficiently explored. One fundamental problem is that the substituted judgment standard, as commonly formulated, is indeterminate in content and thus offers the surrogate little or no guidance. What the standard does not specify is just how competent one should imagine the patient to be, and what else one ought to envision about the patient's hypothetical outlook and the circumstances surrounding his or her decision making. The article discusses this problem of underdetermined decision conditions.