The Ship of Theseus

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.

The Ship of Theseus is a thought experiment in the field of Metaphysics of Identity which asks the question of whether an object that has had all of its components replaced remains fundamentally the same object.

In philosophy of mind, the ship is replaced by a person whose identity over time is called into question.

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In ontological engineering such as the design of practical databases and AI systems, the thought puzzle appears regularly when data objects change over time.

There are four proposed “resolutions” to the thought experiment and those include:

No identity over time §

• Proposed by Heraclitus who attempted to solve the problem by using a metaphor of a river where water replenishes itself as it flows past
• “Upon those who step into the same rivers, different and again different waters flow”
• Plutarch disputed Heraclitus’ claim about stepping twice into the same river, citing that it cannot be done because “it scatters and again comes together, and approaches and recedes”

Four-dimensionalism §

• Ted Sider and others have proposed that considering objects to extend across time as four-dimensional causal series of three-dimensional “time-slices” could solve the ship of Theseus
• All four-dimensional objects remain numerically identical to themselves while allowing individual time-slices to differ from each other
• The aforementioned river, therefore, comprises different three-dimensional time-slices of itself while remaining numerically identical to itself across time
• One can never step into the same river-time-slice twice, but one can step into the same (four-dimensional) river twice

Cognitive science §

• Chomsky argued that the problem exists due to externalism: the assumption that what is true in our minds is true in the world
• Cognitive Science would consider The Ship not as a thing, nor even a collection of objectively existing thing-parts, but rather as an organisational structure that has perceptual continuity
• When Theseus thinks of his ship, he has expectations about what can be found where, how they interact, and how they interact with the wider world. As long as there is a time/space continuity between this set of relationships, it is The Ship of Theseus
• An organisational structure of course has to have components, but these also are defined in the same way