Misjudgments concerning the efficacy of therapies for purely epistemic reasons are not the only worry that one might have about expert opinion. Medical experts and patients are in what economists call a principal-agent relationship. The principal—in this case, the patient—desires the delivery of a certain good or service—in this case, his health. He instructs an agent—in this case, the doctor—with it, because he lacks the expertise to produce the good himself. The good can only be produced with