12. It is the corporeal, then, that demands magnitude: the Ideal-Forms of body are Ideas installed in Mass.
But these Ideas enter, not into Magnitude itself but into some subject that has been brought to Magnitude. For to suppose them entering into Magnitude and not into Matter — is to represent them as being either without Magnitude and without Real-Existence [and therefore undistinguishable from the Matter] or not Ideal-Forms [apt to body] but Reason-Principles [utterly removed] whose sphere could only be Soul; at this, there would be no such thing as body [i.e., instead of Ideal-Forms shaping Matter and so producing body, there would be merely Reason-Principles dwelling remote in Soul.]
The multiplicity here must be based upon some unity which, since it has been brought to Magnitude, must be, itself, distinct from Magnitude. Matter is the base of Identity to all that is composite: once each of the constituents comes bringing its own Matter with it, there is no need of any other base. No doubt there must be a container, as it were a place, to receive what is to enter, but Matter and even body precede place and space; the primal necessity, in order to the existence of body, is Matter.
There is no force in the suggestion that, since production and act are immaterial, corporeal entities also must be immaterial.