I did the ferry thing in class, here's what I wrote:
The discussion of the identity of the schooner in class prompted an analysis on the meaning of identity. The puzzle of whether S2 or S1 was the original schooner showed that something as fundamental as identity, which most take for granted, has no clear meaning.
My stance is that either both S1 and S2 are potentially the original schooner, and both S1 and S2 are potentially not the original schooner, as the owner would not be able to tell the difference between the two. An object’s identity is by no means tangible. It is something that humans arbitrarily give to the object, and because of this human factor, identity is subjective. One example given in the class discussion was about a valuable vase that I happened to own. If this vase were to be replaced with some replica, would it be the same vase? Barring any knowledge of the replacement, my stance is: yes, it is the same vase. Only when something is observed, and hence known does it become a part of reality. In any case, whether it is actually the same vase or not isn’t relevant, what is relevant to the argument is that the identity of the vase remains the same. As long as I don’t know the vase was replaced, its identity will still remain the same. What reason would I have to change its identity? This thought experiment illustrates that an individual object does not have a tangible, measurable “identity”. Identity is given to objects by people. This means that assuming the owner of S1 and S2 cannot tell the difference between the two, they both either become the schooner or they both are not the schooner. Of course, these two schooners are unique in that they are present in different areas of space. If the owner had foreknowledge of which was which, could one retain the original schooner’s identity while the other did not? No, their identities are still interchangeable because they could simply be switched without the owner’s knowledge and then all of a sudden the other schooner assumes the identity of the original. In a way determined by the owner of the schooners, both S1 and S2 are the original schooner and both S1 and S2 are not the original schooner.
If I had absolute knowledge of which was S1 and which was S2, then I would say that S1, the schooner that had its parts replaced, would retain the identity of the original schooner, simply because its identity came from my memories that I associated with it. As long as S1 was periodically used by me, over the period of two years I would transfer the identity of the original schooner’s parts to the parts of S1. This is opinionated though, because identity is subjective as shown in the vase example. Also it isn’t really relevant because the owner is not supposed to be able to be able to tell the difference.