Substance is the cause of modes. Moduses, or individual transient forms, are the essence of the action of a substance. Thus, the relation of substantiality requires a relation of causality and, further, an interaction. Spinoza’s system outlines a fundamentally correct resolution of the problem of substance, causality, and interaction.
“Interaction,” says Engels, “is the first thing that we observe when we begin to consider moving matter as a whole from the point of view of modern natural science. We observe a number of forms of motion: mechanical motion, light, heat, electricity, magnetism, chemical addition and decomposition, changes in the state of aggregation, organic life, which all — if we exclude organic life — transform into each other, condition each other, are the cause, the action there, and the cumulative sum of the movements with all changes of the form remains the same. Spinoza’s “substance is causa sui” expresses perfectly the interaction.”
Abram Deborin (and Engels) on Spinoza’s materialist and dialectical conception of causality, “Hegel and dialectical materialism”, 1929.