The over-arching purpose of this thesis is to consider how Diderot and Lessing dealt with the impact of Spinozist thought and its consequences and to analyse the extent to which they can be said to exemplify a post-Spinozist mentality in dealing with three main issues: virtue and vice; freedom; and, finally, belief. It aims to prove not that Diderot and Lessing were direct disciples of Spinoza, but that both were influenced by the post-Spinozist climate pervading the eighteenth-century intellectual world. At the same time as locating similarities of approach between their thinking and Spinoza's, this thesis will also point out how Diderot and Lessing went beyond the Dutch philosopher's thinking, and how they often showed themselves to be even more radical than Spinoza himself.

The thesis examines the linkage between ontology and politics in Spinoza, and considers the extent to which his philosophy discloses novel materialist conceptions of nature, history and society. It explores the distinct paradigm of the individual proposed by Spinoza emerging from his materialist ontology, and the ways in which this impacts effectively upon the constitution of the multitude as a political category. Arguing that Spinoza’s ontology unveils a more complex process of vital and psychic individuation, I develop a contemporary interpretation of Spinoza’s writings through Simondon’s notions of collective being, disparation, emotions and transindividuality. The study of Spinoza’s ontology in the light of Simondon is crucial for re-considering the central role of affectivity within the genesis and development of human beings. This refers to the redefinition of affectivity as a powerful source of psychic and political individuation, which is the cornerstone of relation, power and transformations. The understanding of Spinoza’s process of affective and collective individuation constitutes the basis for analysing his political theory. The inquiry focuses to the emergence of the political status of the multitude from this complex process of collective and affective individuation, and considers the extent to which the multitude impacts concretely upon the realm of the political. Specifically, the discussion draws attention to the affective state of the multitude, and the ways in which this produces fundamental relational events, meanings, power and problematic political individuals. The argument then turns to examine the model of democracy proposed by Spinoza and the role of the multitude within the constitution of the democratic body. It sheds light on the pivotal part played by the multitude within the production of democracy, and investigates the interface between affectivity and democracy more broadly.