The essay takes its point of departure from Monod's reading of dialectical materialism in Chance and Necessity. A passage of Engels's Dialectics of Nature, which identifies Spinoza's concept of causa sui with the Hegelian concept of interaction [Wechselwirkung], provides the opportunity to examine the consequences of Monod's claims more closely. Using Spinoza's philosophy as a litmus test, the essay attempts to demonstrate the debt of Engels's materialism to Hegel's Science of Logic by tracing the development of the concept of Wechselwirkung (...) in classical German philosophy. A profound difference between the Spinozan and Hegelian concepts becomes apparent: while the concept of Wechselwirkung implies a totality present to itself as simultaneity, permitting the flow of a linear, homogenous and empty time upon which stages of development can be inscribed, the concept of causa sui implies a totality without closure, a totality whose eternity is identified with the necessary and infinite network of modal durations. The essay concludes by suggesting that Spinoza's concept of causa sui allows us to rethink the relation between freedom and necessity in the Marxist tradition in conjunctural and aleatory terms. (shrink)