Traditional labels for central tenets of Spinoza's metaphysics support the view that, in response to Descartes, Spinoza maintains a strict symmetry between thought and extension. Descartes had argued that existence includes some things that are minds, others that are bodies, and still others that are both minds and bodies. Thus God and angels are minds; a chair is a body; and I, in at least one important sense, am both. In what has been known as the ‘dual aspect theory’, Spinoza rejects this view. He contends instead that all things, including God and ordinary objects and all persons, are in one way thinking and in another way extended. In the case of a person, for example, I am a mind; I am a body; and my mind and my body, although they are of different attributes, are identical. In his accounts of causal relations, Descartes had argued that mind can interact with mind in isolation from all body; that body can interact with body in isolation from all mind; that minds can affect bodies; and that bodies can affect minds. The causal history of the world for Descartes, then, might include chapters that are wholly mental, chapters that are wholly corporeal, and chapters that include interaction between mind and body. In what has been known as his ‘parallelism’, Spinoza rejects all mind–body interaction and maintains, moreover, that the causal interactions of bodies are in some sense the same as the causal interactions of minds and vice-versa. Bodies do affect other bodies and minds affect other minds. Mind and body are parallel in the sense that the order of those causal interactions is the same: there is only causal history of the world.
The dual aspect theory and parallelism undoubtedly offer advantages over their Cartesian rivals, notably in the accounts that they offer of the mind–body relation in human beings. Spinoza's views raise problems of their own, however. A particularly pressing problem concerns causation among finite things. Ordinarily we might suppose, whenever A causes C and B is identical to A, that B is also the cause of C. For example, if Lorde has inspired the youth of the world, and if Lorde is identical to Yelich-O’Connor, then it just seems clear that Yelich-O’Connor has inspired the youth of the world.