The independence of the attributes in Spinoza

According to Spinoza, all of reality—namely, deus sive natura and its modes—can be independently understood under each of (at least) two attributes: thought and extension. Under the attribute of thought, we have a world of ideas, and under the attribute of extesion, we have a world of bodies. There is identity between the two worlds: each idea is about a body. We have a beautiful account of the aboutness relation: the idea is identical to the body it is about, but the idea and body are understood under different attributes.

But here is a problem. It seems that to understand an idea, one needs to understand what the idea is about. But this seems to damage the conceptual independence of the attributes of thought and extension, in that one cannot fully understand the aboutness of the ideas without understanding extension.

I am not sure what to do about this.

Inductive evidence of the existence of non-spatial things

Think about other plausibly fundamental qualities beyond location and extension: thought, charge, mass, etc. For each one of these, there are things that have it and things that don’t have it. So we have some inductive reason to think that there are things that have location and things that don’t, things that have extension and things that don’t. Admittedly, the evidence is probably pretty weak.

Two conceptions of matter

The philosophical tradition contains two conceptions of matter. One kind, associated with Descartes, connects matter with space: matter is what is responsible for spatial properties like extension or location. The other, associated with Aristotle, connects matter with passivity: matter is what makes an entity have a propensity to be the patient of causal influences. The spatial conception of matter has been the more popular one in recent times. But here is a reason not to go for the spatial conception of matter. The concept of materiality seems fairly close to the fundamental level. But it may well turn out--string theory is said to push in that direction--that at the fundamental level there is no such thing as space or time or spacetime. If that is a serious epistemic possibility, it would be good to do more work on the Aristotelian option.

Extension and mereological universalism

Plausibly, a fusion of extended objects is extended. Also, plausibly, an extended object has a size. Now suppose, as is surely possible, that there are two universes that aren't spatiotemporally connected, and an extended object A in one and another extended object B in another. Then the fusion of A and B would be an extended object that has no size, since there is no meaningful distance between a part of A and a part of B. Hence, given our assumptions about extended objects, mereological universalism--the thesis that necessarily all pluralities have a fusion--is false.