Since the development of possible interventions on brain mechanisms is rapidly growing we are confronted with the ethical question whether we can reasonably allow all the interventions that science makes available. Are there any constraints if we no longer accept arguments from tradition or autority? We can identify two groups of constraints: one is the obvious constraint that guides any ethical consideration. We cannot allow interventions if a systematic use of them would severely damage our society. The guiding principle underlying this constraint is basically some utilistic evaluation process (typical for any ethical decision) in which we have to account for subjective preferences, on the one hand, and objective needs in a human society, on the other. The second constraint is related to an assessment of the (level of) autonomy that is involved in decisions about brain interventions. Instead of discussing cases of extreme pathology, let us consider an everyday situation. If I want to have a brain intervention that changes my personality now: is it reasonable that I receive such a treatment? The key to answering this question can be found in an assessment of the autonomy that is involved in this decision. This leads to the key question of this paper: To which extend are we autonomous and how can we evaluate the autonomy of a person? I develop an outline of an answer to both questions illustrating the limits of autonomy and the limits of self-determined changes of personality.