  One of the important things that Spinoza does in the first two parts of the Ethics is to establish Extension as a divine attribute (elements of this view are evident already in KV I/25). Although Spinoza adopts many important aspects of Cartesian metaphysics, he collapses the divide between the infinite and created substances. This means that principal attributes that were at the “created substance” level in the Cartesian set-up are “moved up”, so to speak, to the infinite substance level for Spinoza. One of these attributes is, of course, Extension. Spinoza has to explain to a resistant audience how Extension can be considered a divine attribute.