The Ship of Theseus raises the problem of identity. The question it poses is whether a ship would still be the same ship if we keep replacing parts of it; such that, in the end the ship is all new. The similar paradox also applies to living things and humans. Suppose our friend John lose his arm, would he still be John that we know? Certainly. Losing his leg? Of course he is still John. But when some parts of his brain were lost and caused the memory loss, we can’t help feeling that he is no longer John.
From my point of view, the paradox arises from combining incompatible concepts.
The question of this paradox, “When would the ship stop being the same one as we keep replacing parts of it?”, mixes the following incompatible concepts together:
By no means these concepts are incorrect individually, but the way we introduce it in the same sentence such that it appears to be a valid question, causes the paradox.
The whole consists of parts
This is a scientific concept invented by our mind. It is objective, just like F = ma. After all, scientific concepts are our attempts to model the world objectively. This statement looks so easy that people will take it for granted, and not view it as a scientific concept at all, but it is not hard to imagine some species that doesn’t have the ability to understand this.
Also, you may argue that not all things can be divided into parts, and bring up your favorite philosophy against this statement. However, the validity of this concept is not important here, since the question already assumes its validity! If you haven’t noticed yet, consider “… keep replacing parts of it?”, it is clear we assume an object in our consideration, which is a ship, consists of parts (and we’re replacing them).
There is nothing wrong with assuming any object consists of parts, we do it frequently.
The concept of a ship, or whatever objects that we refer to every day such as a car, a bottle of water, a man, a dog, is not trivial at all. They are subjective and their meanings depend entirely on the individuals perceiving. Take any basic word, like ‘dog’, and this would stimulate a variety of past memories between each of us about dog, as we have different experiences toward the word. A dog lover might think of dogs at his home the moment he heard the word, while I have a very general mental image about dogs around the street.
The ability to name our personal experiences and sense data into groups. Some groups have words for them, such as ship, iPhone and Steve Jobs. There are also groups without words. Not everything in our mind must have a word for it. This grouping is done by a system in our brain, and it is unnecessary to know what it is exactly. Whatever it is, the subjectivity of relations between experiences and groups in each person is achieved by this system.
One important characteristic of groups is that each refers to specific experiences. Dalmatian and dog are completely different things. There is no relation between them in this sense. For example, when referring separately, the first thing that comes to mind when we hear the word ‘dalmatian’ and ‘dog’ are different. While ‘dalmatian’ raises a particular mental picture, ‘dog’ might raise a very general, abstract idea of dogs.
You may argue that there is an obvious relation between them, that dalmatian is a kind of dog. Again, we must acknowledge that this kind of statement is objective. In case you have not seen it, there is a concept of generality and specificity, or, one concept is a kind of another concept. This is one of the human’s approach to manage the chunk of concepts, just like a division.
A group refers to specific experience and gives rise to our subjective behavior related to naming things. Because a group is the final product of this system presented to the mind, there is no further divisions or any other kinds of modification after this point.
From this, it is easy to see that the question “when would the ship stop being the same one as we keep replacing parts of it?” doesn’t make any sense. A ship, a human or any objects, are concepts in our mind and are all subjective. “A whole consists of parts” is objective. Combining two of them certainly causes confusion.
In general, my response to the paradox is that a concept of identity makes no sense when you try to think about it objectively.