Let's go back to the sock. Sure, the sock is showing holes, and stitching up its weak places keeps it functional. But our minds might more profitably be thinking about the mystery of this formal principle that endures through material substitutions. Surely the lasting strength of character counts as much as the durability of wool. Sometimes the stitchings and darnings don't take.
Medicine watches carefully for rejection after transfusions, organ transplants, and bone grafts.
The formal principle that guarantees sameness despite the introduction of exotic material is named by medicine the immune system. This system accepts or rejects replacements in accord with its own innate code. The new materials must be integrated into the integrity of the person. Or, as they might have said in church debates nine hundred years ago, the material must be accommodated to the form. It must fit my innate image.