THREE-DIMENSIONALISM RESCUED:A BRIEF REPLY TO MICHAEL DELLA ROCCAIn an important and subtle article,1 Michael Della Rocca offers a new characterization of 3dism and 4dism in order to break the impasse between them.2 On the new characterization, 3dism is distinguished by a commitment to primitive persistence; 4dism is not. Focusing only on part of his long and complex discussion, I want to isolate Della Roccas argument that 3dism is incoherent, with two aims in view: First, to show that his argument against 3dism is unsound; and second, to rescue 3dism as newly characterized3 from Parfits Plausible Principle (PPP). If I am successful, 3dism will remain as secure as ever. There are two main ingredients of Della Roccas argument against 3dism: First, there is the idea of primitive persistence. Let primitive persistence name the view that there is no independent explanation of the trans-temporal identity or nonidentity of an object. On 3dism, “the persistence of the object is explained in part in terms of the persistence of the object itself. Thus, the persistence of the object is not really explained 1 Michael Della Rocca,“Primitive Persistence and the Impasse between Three-Dimensionalism and Four-Dimensionalism” this JOURNAL CVIII,11 (November 2011): pp. 591-616. Hereafter, page numbers referring to this article will appear in parentheses in the text.2 I follow Della Rocca in using the terms “3dism”, “4dism”, “3dist” and “4dist” in obvious ways.3 Officially, the new characterization applies to the difference between 3dism and4dism, but Ill also use the term to apply to 3dism as being committed to primitive persistence.1at all; it is primitive.” (596) The 3dist, but not the 4dist, is committed to the claim that “persistence is primitive, inexplicable.” (593) Second, there is Parfits Plausible Principle. Let PPP name the following principle concerning branching and identity, to which, Della Rocca says, both 3dists and 4dists are “deeply committed” (593): (PPP) In a case in which there are objects, A, B, and C, B C, B and C are equallyand significantly causally and qualitatively continuous with A, and there isno other object besides A which exists at the same time as A and which is such that B and C are as causally and qualitatively continous with it as they are with A, then it cannot be the case that A = B and A C and it cannot be the case that A B and A = C. (599)Della Roccas main argument against 3dism can be set out as follows: (606):(a) PPP. (b) PPP commits one to the rejection of primitive persistence in general.(c) 3dism commits one to primitive persistence. (d) 3dism is incoherent.Although valid, this main argument is unsound. Consider an example that Della Rocca himself uses: Swinburnes view of personal persistence in terms of sameness of soul. Call this view sameness-of-soul. Sameness-of-soul poses a dilemma for PPP: Either sameness-of-soul does not violate PPP or sameness-of-soul does violate PPP. As we shall see, either way, one of the premises of Della Roccas main argument is false. Consider the first horn. Suppose that sameness-of-soul does not violate PPP. Indeed, Della Rocca explicitly argues that sameness-of-soul does not violate PPP. If B 2“has” As soul, but C does not, then “it seems natural to say that Bs states are causally continuous with As states in a way that Cs states are not. Thus B and C are not equally causally and qualitatively continuous with A, and so the fact that A = B and A C does not violate PPP.” (601, emphasis his) Not only does sameness-of-soul not violate PPP, but also it does not commit one to primitive persistence, according to Della Rocca. Della Rocca says, “Even on Swinburnes view, persistence and nonpersistence of persons in a case of apparent fission is grounded; in particular, it is grounded in the presence or absence of a soul.” (601) Grounded persistence is incompatible with primitive persistence. So, Della Roccas argument leads to the conclusion that sameness-of-soul is incompatible with primitive persistence.Now we have a counterexample to premise (c): Premise (c) holds that 3dism commits one to primitive persistence, and sameness-of-soul is paradigmatically a 3dist view; so premise (c) leads to the conclusion that sameness-of-soul commits one to primitive persistence. But Della Roccas argument leads to the conclusion that sameness-of-soul is incompatible with primitive persistence, and hence does not commit one to primitive persistence. If Della Roccas argument on the first horn of the dilemma is correct, then premise (c) is false. Consider the second horn of the dilemma. Suppose that Della Rocca is wrong, and that sameness-of-soul does violate PPP. Here is a sketch of how, despite Della Roccas above argument, sameness-of-soul may violate PPP. Construe causal continuity as follows: X and Y are equally causally continuous with Z just in case for each of Zs causally efficacious states, there is a qualitatively indiscernible causally efficacious state 3of X and a q