  In the main article on Leibniz, it was claimed that Leibniz's philosophy can be seen as a reaction to the Cartesian theory of corporeal substance and the necessitarianism of Spinoza and Hobbes. This entry will address this second aspect of his philosophy. In the course of his writings, Leibniz developed an approach to questions of modality—necessity, possibility, contingency—that not only served an important function within his general metaphysics, epistemology, and philosophical theology but also has continuing interest today. Indeed, it has been suggested that 20th-century developments in modal logic were either based on Leibnizian insights or at least had a Leibnizian spirit.