Spinoza on Extension

In the second part of the Ethics, Spinoza claims that “Extension is an attribute of God, or God is an extended thing [res extensa]” (EIIp2, G II 86/C 448). As ‘extendere’ is used from Aristotle’s Greek equivalent ‘επεκτείνεται’ up through Spinoza, for something to be extended simply means for it to be spread out or to have dimensions. So, considering this claim in isolation from the rest of Spinoza’s work, it is natural to think that Spinoza takes corporeal substance, or God, to be something with dimensionality. That might mean that it is something with mere dimensionality, as in Jonathan Bennett’s influential interpretation, on which substance, understood under the attribute of Extension, is identical with space, and modes of physical substance are spatiotemporal regions of qualitative variation. Or it might mean that substance is an extended something, be that something matter, impenetrability, force, power, or whatever. In either case, it suggests that substance has dimensionality. A common worry about such a claim, in Spinoza’s time, was that it seemed to attribute to God a slew of imperfections associated with matter, chief among them divisibility, inertia, and passivity.