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In Greek mythology there was a hero named Theseus, renowned for various acts of heroism both dramatic and pedestrian, but most famous for slaying the Minotaur at the center of the labyrinth at Minos.  Upon his triumphant return voyage to Athens, his ship was moored in the Athenian harbor as a national treasure.  However, rather than being permanently moored and allowed to decay (like many other historical sites) the ship was kept in a seaworthy state so that it could be used to ferry city officials to various festivals on neighboring islands.  Over the centuries, this meant that new ship pieces and hull patches were applied to the wooden ship as parts of it slowly rotted away… with the end result that, one day, every single plank of the ship had been replaced.  In the minds of the Athenians (and a vast multitude of philosophers since) this raised a most intriguing question: since every last part of the Ship of Theseus has now been replaced, is the boat moored in the harbor of Athens actually the Ship of Theseus?  Are the ship of legend and the ship today “the same” ship?

While many resolutions to this problem have been posited, for objective identity – the collective physical properties of an object at both a given moment in time and the aggregate history of that object from existence to non-existence – the four-dimensionalism explanation is most accurate, as well as being most useful.  However, the Ship of Theseus is more an issue of subjective identity – how the relationship between the object and the observer has changed or remained similar from one moment to another.  So while the physical composition of the Ship of Theseus has quantitatively changed between the Ship now and the Ship then, the people of Athens (et. al.) may still recognize the Ship as being the one of legend as the Ship has been a constant part of the observers’ lives while simultaneously undergoing physical change.

I believe the question of identity may be resolved by considering the interaction of the object with its surrounding reality.  To create a more contemporary metaphor: my first vehicle was a 1979 Ford Pinto station wagon.  It functioned, but only just; over the course of our history together, I had to replace various parts as they broke (or came on sale); a piston here, a door handle there, eventually the seats and steering wheel as well.  One day, looking back over my Pinto-related expenses, I came to the conclusion that every last part of my Pinto had been replaced… but despite this, I still referred to the vehicle thus created as my Pinto.  I have come to the conclusion that the reason this is so is that I was able to update my subjective identity for my Pinto – which I shall refer to as an “identity image” – on a regular basis.  Since I was a party to the changes and replacements made to the vehicle, each physical (objective) change resulted in an accompanying (subjective) change in the “my Pinto” identity image held in my head.  This rate of change of the identity image for an object is what cues our senses to changes over time; if I have two cousins, one of which I grew up next door to and another I haven’t seen since I was ten, meeting both of them for coffee (or seeing them on Facebook) now that I’m twenty-seven will result in my being much more surprised by the (objective) changes in the cousin with which I am less familiar than the one with whom I grew up.  Upon meeting them both, the identity image I had of the one was much more recent than that of the other.  So my Pinto remains “the same” over time because each little objective change was immediately incorporated into my Pinto’s identity image.

The philosophical problem here – one of identity image and “sameness” – is not purely philosophical, as it has many real-world implications as well.  For example:  in the Ray Bradbury short story The Utterly Perfect Murder, the protagonist plans to return to his childhood home and murder his childhood bully.  Enroute, he reviews the multitude of small evils visited upon his person by the bully, building up justification in his mind and hardening himself to his purpose.  But upon arriving at his bully’s front door, the protagonist discovers a shrunken, forlorn, decrepit old man, ground into dust by a lifetime of hardship… while the protagonist himself is healthy, vibrant, and fully engaged with his life.  At that moment, seeing that his bully is no longer the man from his past, the protagonist’s purpose evaporates.  The person he went home to kill is already dead, replaced by the completely different individual standing before him.  Seeing that life has accomplished his task for him, the protagonist departs in peace.

Comparisons are made constantly between the object we remember and the object we see before us; the identity image is regularly consulted and updated.  While objective differences are helpful indicators of appropriate behavior, it is the subjective identity that regulates our interactions with others.  In particular, humans have a tendency to gravitate towards people of which we have a positive subjective identity image… regardless of whether the objective identity supports this image.  So the best response to both objective change and perceived change would seem to be one of cautious curiosity; openness to the idea of change being a constant and multifaceted thing, combined with a sense that one’s perceptions may differ from those of another person.  This would insure not only that one’s identity image is correctly aligned with one’s perceptions, but also that one’s perceptions do not lead one’s thinking… as doing so leads to misunderstanding.  The entire point of communication can be seen, in this light, to be the purposeful alignment of two differing identity images… that of the observer, and – perhaps uniquely human – that of the observed.