The Ship of Theseus Puzzle

Thought experiments play various roles in philosophy. Often, they have anargumentative function: The judgments they elicit bear on some philosophical debate. The Gettier case, theGödel case, the Twin Earth case, the Frankfurt case, etc., illustrate the argumentative function ofthought experiments. Much of recent metaphilosophy (e.g., Williamson, 2007; Machery, 2017) examines whether and how thought experiments can fulfill this argumentative function. But thought experiments also have less controversial functions. Sometimes they are just meant to illustrate a definition or a theory: Arguably, Davidson’s swampman case is only meant to illustrate (not to support) the proposition that the content of thoughts depends on historical facts.Another function of cases is to provoke the reader, that is, to elicit puzzlement in order tomotivate philosophical inquiry. Metaphysical cases such as the statue of clay case are often meant to fulfill this provocative function. To fulfill a provocative function, a thought experiment must meet the following condition (which we will call “Ambivalence”): Readers should feel inclined to assert two prima facie inconsistent propositions. This ambivalence is instrumental in leading readers to philosophizeabout the philosophical issue raised by this thought experiment (be it identity, persistence, constitution, etc.). Ambivalence refers to a psychological fact—that is, it is a psychological factthat readers are so inclined—and psychological methods can be used to assess whether a thought experiment successfully provokes. A thought experiment fails to fulfill its provocative function if it elicits a single, obvious answer. If a provocative thought experiment is meant to provoke not just readers from a particular cultural background, but all or most readers, it must fulfill a second condition (which we will call“Universality”): It must elicit an ambivalent state of mind in readers of all demographic, particularly of all cultural, backgrounds. In this article, we examine whether one of the most venerable thought experiments inmetaphysics, the Ship of Theseus case, successfully fulfills its provocative function. The Ship ofTheseus case is an ancient puzzle about persistence. It emerges in partial form in the writings ofthe Greek biographer Plutarch (1914) and is fleshed out in its modern form by Hobbes (1839): We will remain neutral about whether the Ship of Theseus case also has an argumentativfunction and about whether it successfully fulfills it. For if, for example, that ship of Theseus, concerning the difference whereof made by continued reparation in taking out the old planks and putting in new, the sophisters of Athens were wont to dispute, were, after all the planks were changed, the same numerical ship it was at the beginning; and if some man had kept the old planks as they were taken out, and by putting them afterwards together in the same order, had again made a ship ofthem, this, without doubt, had also been the same numerical ship with that which was at the beginning; and so there would have been two ships numerically the same, which is absurd. (De Corpore II, p. 11)The issue is this: On the one hand, it seems that the Ship of Theseus can survive thegradual replacement of parts and so it seems that the ship made by gradually replacing the parts(we’ll call it “Replacement”) is indeed the original ship. On the other hand, when all of theoriginal parts are assembled in the same form as the original ship, it seems that the ship made from the original parts (we’ll call it “Original Parts”) is indeed the original ship. Both can’t be the original ship. So which one is the original ship—the Ship of Theseus— Replacement orOriginal Parts?Many philosophers have viewed this case as presenting a genuine puzzle arising from two opposite inclinations to judge: The “continuity of form” between the original ship and Replacement leads us to think that Replacement is the original ship, while the “continuity ofmatter” between the original ship and Original Parts leads us to think that Original Parts is theoriginal ship. These two criteria for reidentifying objects pull in opposite directions (Rea, 1995, p. 532; see also e.g., Hirsch, 1982; Hughes, 1997; Lowe, 1983; Nozick, 1981; Scaltsas, 1980;Sider, 2001; Simons, 1987; Wiggins, 1980). Some philosophers who think the Ship of Theseus case presents a genuine puzzle aboutidentity even doubt that the puzzle has a solution. For instance, Scaltsas (1980) claims that "the example of Theseus’s ship . . . [is an] actual paradox.... [T]here is no sharply defined hierarchy of sufficiency conditions [for artifact identity], so that in cases of conflict we are not always in a position to determine whether the new object is identical to the initial one or not. The reason is that the cases of conflict are so rare in everyday life ... Hence, our intuitions are blunt when it comes to making such decisions" (p. 152). In a similar vein, Wiggins (1980) claims that the Ship of Theseus case is “irreclaimably paradoxical” (p. 97). By contrast, other philosophers deny that the Ship of Theseus case presents a genuine puzzle. Smart, in particular, holds that thinking that the continuity of matter criterion for identity is important has led to “false beliefs—(1) that this condition [i.e., the continuity of matter criterion] applies to the Ship of Theseus case and (2) that it either outweighs or is outweighed by the continuity of form condition” and this has “been responsible for generating a puzzle where no real puzzle or need for a decision exists”(Smart, 1973, p. 27). The “obvious solution,”according to Smart, is that Replacement is the original ship and the “existing rules of identity”prove to be “perfectly adequate for this unusual case” yielding “a non-arbitrary and clear-cutdecision” (Smart, 1972, p. 148).22 It is not entirely clear how to understand Smart’s claim that the Ship of Theseus puzzle has an“obvious solution.” An anonymous reviewer points out that Smart’s claim may not be about ourjudgments about persistence: It may not be a psychological claim. Rather, Smart may be merelysaying that one of the two options is clearly the correct one. We believe that Smart’s claim thatthere is an “obvious solution” can be understood in several ways, including in a non- Our goal in this article is to examine whether the Ship of Theseus case is a genuin puzzle that can fulfill the provocative function. We won’t address the question of how objects actually persist through part alterations. To use the terminology of Machery (2017), we are not concerned with the material problem of persistence. Nor will we examine the metaphilosophical question of whether the judgments elicited by the Ship of Theseus case can somehow be brought to bear on philosophical theorizing about identity. Instead, we examine whether Ambivalenceand Universality hold for the Ship of Theseus case, i.e., whether the Ship of Theseus case elicitscontradictory inclinations to judge and whether it does so across demographic groups.