 Spinoza’s defense of (15) would likely be to appeal to the self-containment of each attribute (Della Rocca 2001). Like substance, each attribute is a wholly self-contained and self-sufficient way of conceiving substance (Ip10). Given such rigid containment, Spinoza would press, how could the fact that a substance has one attribute preclude that substance from having additional attributes? If there are no explanatory or conceptual relations between the attribute of thought and the attribute of extension, the fact that a substance is thinking cannot explain why that substance could not also be extended. The lack of a reason for excluding an attribute is part of Spinoza’s reason for including all possible attributes as attributes of God. However, even if he can defend the rigid self-containment of attributes, he will also need a positive reason for including them all together as attributes of a single substance.