 Spinoza holds not only that the existence of things must be explained, but also that the coherence, or incoherence, of their essences (what others would call their possibility) must be explained (E1p33s1). Similarly, the essences of things must also have a cause (E1p25). In general, many commentators have thought that Spinoza relies, either explicitly or implicitly on the PSR to motivate many of his most important and innovative doctrines, such as the Identity of Indiscernibles (E1p4. Although see Lin forthcoming for a dissenting view), substance monism (E1p11 and E1p14), and the rejection of free will (E1p32 and E2pp48–49). In E1p21d Spinoza relies on the PSR to infer a bold causal principle: a simple cause has one, and only one, simple effect. Had the cause more than one effect, the difference between the two effects would be unexplainable insofar as each effect is supposed to be fully explained by the same cause. Thus, if we experience a cause bringing about more than one effect, we should conclude that that the cause was not simple, but comprised of parts (so that the different parts contributed to the causation of the different effects. See Melamed 2013a: 117–119, and Melamed 2013b: 212–213.