In order to protect patients against medical paternalism, patients have been granted the right to respect of their autonomy. This right is operationalized first and foremost through the phenomenon of informed consent. If the patient withholds consent, medical treatment, including life-saving treatment, may not be provided. However, there is one proviso: The patient must be competent to realize his autonomy and reach a decision about his own care that reflects that autonomy. Since one of the most important patient rights hinges (...) on the patient's competence, it is crucially important that patient decision making incompetence is clearly defined and can be diagnosed with the greatest possible degree of sensitivity and, even more important, specificity. Unfortunately, the reality is quite different. There is little consensus in the scientific literature and even less among clinicians and in the law as to what competence exactly means, let alone how it can be diagnosed reliably. And yet, patients are deemed incompetent on a daily basis, losing the right to respect of their autonomy. In this article, we set out to fill that hiatus by beginning at the very beginning, the literal meaning of the term competence. We suggest a generic definition of competence and derive four necessary conditions of competence. We then transpose this definition to the health care context and discuss patient decision making competence. (shrink)