Firstly, we named them. Hence, we perceived them. To name them, we associated them with their jobs as crudely as we understand them. Kidneys regulate blood, hearts pump it all throughout, etc. At that, I propose that the beginning of identity is in the naming. When we accept something with whatever personal associations we create toward it, we give it an identity. From this point of view, however, identity is relative because after all, what if the Kidney never thought of it as a Kidney, but instead a city? Maybe it's a city of clustered cells. We never really know. We just imposed upon its identity by calling it as such
That's an interesting point, Min. I understand it as saying that the concept we have of 'kidney' is a matter of convention. It is not something transcendental, which exists outside of space and time, and which we have happened to uncover. Rather communities of human subjects interact, communicate, and intersubjectively come to an agreement that the name 'kidney' will refer to a concept with meaning
x. Both the meaning of the concept and its name are, in some sense, arbitrary, since they have been conventionally defined by the community. Note however that the result is not subjective. It is objective in a social sense: it has been agreed by a community of human subjects.
I agree with all of this. Now the interesting question is: in this intersubjectively agreed conventional concept of 'kidney', are there elements that are actually
not a matter of convention? Here two candidates appear: essence and identity. The philosopher of language will be tempted to say that 'essence' is just another matter of convention (which seems to be the route you've chosen, too). But it is not so evident with identity. Like you suggested, it does
not seem to be a mere matter of convention when my kidney stops being
my kidney and becomes
John's kidney. It does not seem to be a mere matter of convention when a body stops being a body and becomes a corpse.
Here, as you suggested, the agency of physical death does seem to have explanatory significance. But what about the suggestion I made in an earlier post:
It seems that most people, on the strength of their intuition alone, would say that Boat A is the boat whose parts got replaced over time. In other words, it remained the same boat. The boat that got assembled years later from the original parts is not Boat A, but Boat B.
The question, of course, is how to provide an argument to support this basic intuition. The argument I have in mind relies on the concept of spatio-temporal continuity (or continuity of space-time paths). An object in spacetime undergoes change, necessarily, but its identity is preserved through change as long as it showcases continuity of space-time paths. Merely replacing a part every month or so does not create a fundamental discontinuity. On the other hand, in the case of the boat which gets assembled years later from the (now cured) discarded original parts, there is a fundamental discontinuity, in the sense that the discarded parts lie scattered in a pile inside a warehouse for years before they are re-assembled. Those years constitute the spatio-temporal discontinuity, and hence the non-identity of the reassembled boat with Boat A.
Could
spatio-temporal discontinuity be the primary cause of identity change? Death would represent an example, but being separated from a body (and added to another) would be another example.