Descartes, Spinoza, Leibniz: An investigation on their understanding of substance

Descartes, Spinoza and Leibniz are generally considered to be united by their rationalist epistemological programme. However, this is not the only possible context in which these philosophers can be considered together. Although not as much studied as the rationalist epistemology, Descartes, Spinoza and Leibniz also meet on a common ground in ontology, namely on the doctrine of substance. The traditional doctrine of substance, based on Aristotle's investigations of being, was largely accepted by his medieval commentators and remained in continuity. Descartes, Spinoza and Leibniz also constructed their philosophies of being on the basis of substance. However, although they share similarities with the traditional doctrine of substance, these thinkers' doctrines of substance diverged from it at many points and thus formed the framework of a modern conception of being based on substance. In this study, Descartes, Spinoza and Leibniz's conceptions of substance are analysed in order to reveal this modern conception of being based on substance as well as to determine its structure and limits. Firstly, Descartes' doctrine of substance is addressed in a historical context and it is attempted to show that this doctrine was created for a philosophical purpose. It is emphasised that Descartes offers two different definitions of substance based on the criteria of being a subject and independence, but the main distinctive aspect of his doctrine appears with the definition of substance based on independence, which is also related to infinity. Descartes' thinking substance was analysed on the basis of his theory of ideas and the faculties of thought, and it was argued that while reducing the scope of the soul he also expanded the scope of thought. Finally, it is stated that the acceptance of the essence of the corporeal substance as space determines the type of mechanism adopted in physics. In the second part, it is argued that Spinoza's doctrine of substance is among the doctrines of substance based on the criterion of independence and that it differs from other doctrines of the same category by defining a conceptual independence in addition to ontological independence. In addition, the uniqueness of his treatment of the essence of substance with the concepts of power and infinity is also emphasised. Spinoza's connection between substance and attribute is discussed on the basis of the problem of unity and multiplicity, and attention is drawn to the shift in perspective that the infinity of these elements may require. Furthermore, the expressive relation defined between substance and attribute and the dynamic aspect of this relation are stated to be the contributions of Spinoza's novel perspective to the ontology of substance. Then, alternative interpretations of the relation between substance and mode are discussed and it is argued that the settlement of this debate may depend on the choice of the criterion of substance. In the last part, where the determinations of the substance are discussed, it is argued that Spinoza does not reduce God to nature, but extends nature towards divinity with the concept of Natura naturans. In the third section, Leibniz's doctrine of substance is analysed under three headings: analytic, dynamic and monadic conception of substance in a chronological order. Leibniz's notion of the individual concept of substance, which constitutes the core of his analytic view of substance, is discussed in connection with his theory of truth, and the transformation of the relation between substance and mode into a necessary bond is highlighted. It is argued that Leibniz's dynamic view of substance, which is based on the definition of substance as activity, emerged as a result of his criticism of Descartes' corporeal substance. Finally, Leibniz's monadic conception of substance based on unity and simplicity, the infinite hierarchy of being and the worldview grounded on it are analyzed in this section. In the final section, it was concluded that although having similar definitions of substance, Descartes, Spinoza and Leibniz developed different ontologies. Despite their differences, it was seen that these three thinkers thought of the substance in relation to infinity and God. It has been argued that a new understanding of substance belonging to the early modern period different from the traditional one can be distinguished and that one of the major contributions of this modern understanding to the ontology of substance is the introduction of the category of mode.