Informed consent is valid only if the person giving it is competent. Although allegedly informed consents are routinely tendered, there are nonetheless serious problems with the concept of competence as it stands. First, conceptual work upon competence is incomplete: the concept is unanalyzed and no logic of competence has been identified. It is thus virtually impossible to reliably discern who is (not) competent. Traditional work on competence has explicated three dichotomies from which the necessary conditions for the possibility of competence will be identified, viz., that competence (1) is either a global or a specific notion; (2) is present in varying degrees or in virtue of a threshold; (3) and may or not include appeals to consequences. Past efforts have failed to notice a fourth dichotomy: competence as an affective or cognitive notion. Failure to designate the appropriate arms of these dichotomies is responsible for there being, at present, no reliable test for competence. The inadequacies of a highly regarded particular test for competence, the mental status examination, are examined in light of this failure. Competence is located within the rule-governed practice of informed consent. That practice and hence, competence, is justified through a Kantian analysis of respect for persons. This analysis reveals that the logic of competence requires assessment in terms of (1) specific rather than global concerns; (2) degrees of ability rather than a threshold; (3) both cognitive and affective abilities; and (4) no appeal to consequences. Based upon the analyses of consent as an example of decision making within the practice of informed consent and of respect for persons, the capacities which jointly comprise competence are identified. These are the capacities to receive, recognize, and retrieve information; to reason about, relate to oneself, and rank options; to choose among alternatives; and, under certain circumstances, to defend one's choice. Identification of the logic of competence and explication of and justification for the capacities that comprise competence to consent provide a conceptual foundation for the crucial concept of competence that the informed consent process currently lacks. It should facilitate construction of a test for competence.