While Spinoza, rejecting the project of “theodicy”, insists on “absolute necessity”of the world from the view point of eternity, Leibniz, as the originator of the concept of “possible worlds,” advocates the optimism, namely the logical contingency and moral necessity of the best of this world. Given this seemingly fatal opposition of two 17th century major metaphysicians about modalities, it is philosophically important to see the causes of this tension and, thereby to have some prospect for better understanding of the problems of modalities.
Firstly, from the representation of recent interpretations of “the necessity of finite modes” in Spinozaʼs Ethica, especially from Huenemannʼs about “the instantiation of geometrical essence” in the finite modes; secondly from contextual understandings of Leibnizʼs comments about texts such as IP29 of Ethica; and thirdly,characterizing the distinction between modal inferences of consequentiae and consequentis in Leibniz, I maintain that the ontologically irreducible status of agency of actions and the proper concepts of logical contingency turn out to be decisive in the controversy on modalities. Finally, it is argued that the modal sentences as such are seen by Leibniz as a type of reflexive proposition the truth values of which cannot be unconditionally decided.