  For Descartes, then, there is the epistemological claim that perceiving Thought does not involve perceiving Extension and vice versa. Each is explanatorily independent from the other, (although not from God). Spinoza adopts this aspect of Cartesian philosophy and holds, as well, that there is what Della Rocca calls, “a conceptual barrier” between Thought and Extension as Spinoza states in the scholium “i.e., one may be conceived without the aid of the other” (Della Rocca, 1996, 9–17). Spinoza holds Thought and Extension to be explanatorily self-contained. Physical changes are to be understood in terms of other physical items, and ideas are to be understood in terms of other ideas. What is ruled out is what can be called “cross attribute explanations.” For example, explaining the movement of my hand by my desire to move my hand. According to Spinoza, the movement of my hand is to be explained purely physically by alluding to other bodies and their motions, while my desire is to be explained by other desires and ideas. Spinoza makes this very clear in 3P2, its demonstration and scholium: