The boatwright working on the old fishing boat in the yard near where I grew up used to say the boat had been repaired so many times that not one original plank remained, and yet everyone still called it by its first owner's name, still trusted it to hold the same shape on the water it had held for decades. This is the old puzzle dressed in local clothes, the ship of Theseus, asking whether an object that has had every part replaced is still the same object, or whether identity was never really located in the wood to begin with.
My grandfather's body, in the last years I knew him, had replaced itself in the ordinary way bodies do, cells dying and being rebuilt, tissue turning over on its own patient schedule, so that very little of the substance he was made of in his youth remained in him by the time I was old enough to really talk to him. He was still, unmistakably, himself. Not because some fixed material core had survived underneath the turnover, but because the people around him kept treating him as continuous, kept calling him by his name, kept holding him responsible for promises made decades earlier as though the person who made them and the person now honoring them were plainly one and the same.
Heraclitus is usually remembered for saying that you cannot step into the same river twice, because new water is always flowing through it. What I find more interesting than the line itself is what it leaves unsettled. The river keeps its name across the whole span of your life and everyone else's, keeps its banks, keeps its bed, keeps being the specific river you grew up beside even though not one drop of the water you first touched in it is still there. Something is stable enough to justify calling it the same river. It is just not the water.
The boatwright's answer, when I once asked him directly whether the fishing boat was still the same boat, was not philosophical. He said it carried the same nets, the same owner's family, the same route out past the point and back before evening. He was describing a pattern of use and relationship, not a claim about matter. The wood had been Theseus's problem. The boat, in daily life, was never really about the wood.
My grandfather, near the end, did not remember everything he had done. Some of what tied his early self to his later self had to be carried by other people, by my grandmother telling stories he no longer initiated, by documents that recorded decisions he had made and that others now had to honor on his behalf. Continuity, in his case, was not sitting quietly inside him waiting to be found. It was distributed, held partly by memory that lived in other people's heads and partly by responsibilities that outlasted any single version of him and had to be carried forward by whoever was left holding them.
I don't think there is a clean answer to when a repaired boat stops being the original boat, or when a body that has replaced nearly all of itself stops being the person who first inhabited it, and I am suspicious of any account that offers one too easily. What I trust more is the pattern I saw in both cases, that identity across time turned out to depend less on a fixed core of unchanged material and more on an ongoing set of relationships, obligations, and recognitions that other people and institutions kept extending toward the same name. The river is not the water. The boat is not the wood. And whatever kept my grandfather himself through all that turnover was never simply him alone, holding his own continuity together by will, but something closer to a promise the rest of us kept making on his behalf, plank by plank, year by year, for as long as we were willing to keep calling him by his name.