 Recently, Michael Della Rocca argued not only that the PSR “provide[s] the key to unlocking many of the mysteries of Spinoza’s philosophical system” (2008: 9), but that Spinoza requires the reduction of the most basic philosophical concepts to reason or intelligibility. This alleged “double use of the PSR” stipulates (1) that everything must be explainable, and (2) that it should be (ultimately) explained in terms of intelligibility. Hence, according to Della Rocca, Spinoza reduces his major philosophical concepts—existence, causation, rightness, and power—to intelligibility (2008: 8–9). While this is a fascinating and bold reading of Spinoza’s metaphysics, it seems to contradict his crucial doctrine of the causal and conceptual barrier between the attributes (E1p10 and E2p6). The reduction of any non-Thought item to intelligibility (presumably, a feature of the attribute of  thought[6])  undermines the barrier between the attributes, and with it the entire edifice of Spinoza’s ontology (see Della Rocca 2012: 12–16; Melamed 2012a; and Melamed 2013a: xv and 196 n. 84).