Substance, nature, and necessity › Mind and body without two interacting substances

In Part II, thought and extension are attributes of God. A particular human mind and body are not themselves attributes. Proposition 13 identifies the object of the idea constituting the human mind as the actually existing body. The distinction between an attribute and its finite modes is essential here.

Proposition 7 states that the order and connection of ideas is the same as the order and connection of things. Its scholium describes a mode of extension and the idea of that mode as one and the same thing expressed in different ways. Spinoza therefore does not begin with two independent substances and then seek a mechanism through which they exchange messages.

Part III, proposition 2 denies that body determines mind to think or mind determines body to motion or rest. Its scholium explains mind and body as the same thing conceived under thought and extension. Within this account, a bodily process is explained through extension and a mental process through thought. Their relation is not the relation between two separate things causing changes in one another across the attributes.

For the message example, we can describe bodily changes associated with responding and the person's ideas about the message. Spinoza's framework asks us to avoid turning these descriptions into an independent mental agent commanding an independent body. The example supplies no empirical account of a particular brain process. Nor does the metaphysical proposal, by itself, settle modern scientific questions about consciousness.

An objection remains: does describing one thing under different attributes explain the relationship, or merely restate the difficulty? Spinoza's answer depends on the wider account of substance and attributes. Someone who rejects that account may find the identity claim insufficient. The proposal can be understood clearly without presenting it as an uncontested solution to every mind-body problem.