  The metaphysics of the good, as it is conceived in the thesis of the convertibility of the good and being in the medieval doctrines of the transcendentals, was central in the early elaboration of the doctrine. Its central inspiration was Aristotle’s critique on Plato’s Idea of the Good in the first book of the Nicomachean Ethics, which proclaimed the transgeneric and analogical character of the good and opened with an authoritative definition: “the good is that which all desire.” (Arist., Eth. Nic. I, c. 1, 1094a2–3) Medieval authors could rely on Boethius’ De hebdomadibus for an explanation of the goodness of reality, which builds basically on its being created by what is essentially good; the treatise is an exemplary harmonization of the Platonic and Aristotelian traditions, which assert the transcendence resp. the transcendentality of the good. Another important argument to account for the convertibility of being and good is found in Aquinas, who connects the actuality of being with the proper character of the good as that which is desirable – since a thing is desirable only insofar as it is perfect, and is only perfect insofar as it is in act, the goodness of a thing depends on the actuality of its being. Aquinas’ doctrine of the transcendentals also provides a major model to think of the relation between the metaphysical and the moral good, insofar as it is foundational not only for the realm of theoretical reason, but also for the realm of practical reason. In the same way as the first principle in theoretical reason, the principle of non-contradiction, is founded on the concept of ‘being’ as first known in the theoretical realm, the first principle in practical reason, the first precept of natural law: “Good is to be done and pursued, and evil avoided,” is founded on the concept of ‘good’ as first known in the practical realm. (See Kluxen 1964, 93–100, &amp; Aertsen 1996, 326–330.)