Heraclituss river fragments raise puzzles about identity and persistence: under what conditions does an object persist through time as one and the same object? If the world contains things which endure, and retain their identity in spite of undergoing alteration, then somehow those things must persist through changes. Heraclitus wonders whether one can step into the same river twice precisely because it continually undergoes changes. In particular, it changes compositionally. At any given time, it is made up of different component parts from the ones it was previously made up of. So, according to one interpretation, Heraclitus concludes that we do not have (numerically) the same river persisting from one moment to the next.
Plato is probably the source of this paradoxical interpretation of Herclitus. According to Plato, Heraclitus maintains that nothing retains its identity for any time at all:
Heraclitus, you know, says that everything moves on and that nothing is at rest; and, comparing existing things to the flow of a river, he says that you could not step into the same river twice (Cratylus 402A).
But what Heraclitus actually said was more likely to have been this:
On those who enter the same rivers, ever different waters flow. (fr. 12)
On Platos interpretation, its not the same river, since the waters are different. On a less paradoxical interpretation, it is the same river, in spite of the fact that the waters are different. On both interpretations of Heraclitus, he holds the Flux Doctrine: Everything is constantly altering; no object retains all of its component parts from one moment to the next. The issue is: what does Flux entail about identity and persistence? Platos interpretation requires that Heraclitus held what might be called the Mereological Theory of Identity (MTI), i.e., the view that the identity of an object depends on the identity of its component parts. This view can be formulated more precisely as follows:
For any compound objects, x and y, x = y only if every part of x is a part of y, and every part of y is a part of x.
I.e., an object continues to exist (from time t1 to time t2) only if it is composed of all the same components at t2 as it was composed of at t1. Sameness of parts is a necessary condition of identity.
It now seems that if we want to allow that an object can persist through time in spite of a change in some of its components, we must deny MTI. An object x, existing at time t1, can be numerically identical to an object y, existing at time t2, even though x and y are not composed of exactly the same parts.
But once you deny MTI, where do you draw the line? Denying MTI leaves us vulnerable to puzzle cases, the mother of all of which is the following.
The ship wherein Theseus and the youth of Athens returned had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.
Plutarch tells us that the ship was exhibited during the time [i.e., lifetime] of Demetrius Phalereus, which means ca. 350-280 BCE. (Demetrius was a well-known Athenian and a member of the Peripatetic school, i.e., a student of Aristotle. He wrote some 45 books, and was also a politician).
The original puzzle is this: over the years, the Athenians replaced each plank in the original ship of Theseus as it decayed, thereby keeping it in good repair. Eventually, there was not a single plank left of the original ship. So, did the Athenians still have one and the same ship that used to belong to Theseus?
But we can liven it up a bit by considering two different, somewhat modernized, versions. On both versions, the replacing of the planks takes place while the ship is at sea. We are to imagine that Theseus sails away, and then systematically replaces each plank on board with a new one. (He carries a complete supply of new parts on board as his cargo.) Now we can consider these two versions of the story:
Let A = the ship Theseus started his voyage on.
Let B = the ship Theseus finished his voyage on.
Our question then is: Does A = B? If not, why not? Suppose he had left one original part in. Is that enough to make A identical to B? If not, suppose he had left two, etc. Where do you draw the line?
Now we have:
C = the ship the Scavenger finished his voyage on.Our problem is to sort out the identity (and non-identity) relations among A, B, and C. The only obvious fact is that B ¹ C (after all, they are berthed side by side in the harbor, so they can hardly be one and the same ship!). Beyond that, there are two alternatives:
Unfortunately, both alternatives lead to unintuitive consequences.
These results seem as paradoxical as the view that there are no persisting objects.
Conclusion: MTI seems too strong. It denies identity to objects that we think of as persisting through time. But that leaves us with some problems:
In fact, there is a way of describing the case of Theseuss ship that seems to demand MTI rather than STC. Suppose the ship (A) is in a museum, and a clever ring of thieves is trying to steal the ship by removing its pieces one at a time and then reassembling them. Each day, the thieves remove another piece, and replace it with a look-alike. When they have removed all the original pieces, we are left with this situation. There is a ship, B, that is in the museum (made of all new materials), and there is a ship, C, in the possession of the thieves (the original pieces of A now reassembled). Which ship is A (Theseuss original ship)? Surely not B—its just a copy of A, left behind in the museum by the crooks to cover up their crime. It is C that will interest the antique dealer who is interested in buying A, the original ship.
We are still struggling with Heraclituss puzzle.
Go to next lecture
on Parmenides, Stage I
Go back to lecture on Heraclitus
Return to the PHIL 320 Home Page