The article analyses the relationship between rhetoric and didactic in Plato's theory of legislative preamble in the Laws on the basis of a comparison of three texts, the passages in Books IV (718a6–723b2) and IX (857c1–e7) that explain the function of preambles and the specific preamble of the law against the atheists in Book X. In spite of the correspondence between the free physician who explains the patient's illness on the basis of a quasi-philosophical conversation about the "nature of bodies" (Book IX) and the legislator who explains the origin of bodily movements in a philosophical way (Book X), the former cannot be considered as the direct counterpart of the latter: in the first case, the patient collaborates with the doctor; not so in the case of the atheist in his relationship with the legislator. This dissymmetry also justifies reading the Book IX passage as advocating a "legislative utopia"—one which by definition is not realized within the framework of the Laws, and in particular not in Book X.