De Spinoza (1632-1677)

iv. Causal Parallelism
An obvious question to ask at this point is whether it is possible for finite modes falling under one attribute to act upon and determine finite modes falling under another attribute. Spinoza’s answer is an unambiguous no. Causal relations exist only among modes falling under the same attribute. His explanation for this may be traced back to an axiom set forth at the beginning of Book One:
IA4: The knowledge of an effect depends on, and involves, the knowledge of its cause.
Given this axiom, if a finite mode falling under one attribute were to have God as its cause insofar as he is considered under a different attribute, i.e., if it were to be caused by a finite mode falling under a different attribute, then the knowledge of that mode would involve the knowledge of that other attribute. Since it does not, that mode cannot have God as its cause insofar as he is considered under some other attribute. In other words, it cannot be caused by a finite mode falling under some other attribute. When applied to modes falling under those attributes of which we have knowledge – thought and extension – this has an enormously important consequence. There can be no causal interaction between ideas and bodies. This does not mean that ideas and bodies are unrelated to one another. Indeed, it is one of the best-known theses in the Ethics that the lines of causation that run among them are strictly parallel: IIP7: The order and connection of ideas is the same as the order and connection of things. In the demonstration of this proposition Spinoza says that it is a consequence of IA4 and leaves it at that. Nevertheless, it is apparent that this proposition has deep foundations in his substance monism. As thought and extension are not attributes of distinct substances, so ideas and bodies are not modes of distinct substances. They are “one and the same thing, but expressed two ways” (IIP7S). If ideas and bodies are one and the same thing, however, their order and connection must be the same. The doctrine of substance monism in this way insures that ideas and bodies, though causally independent, are causally parallel.