Disregarding completely general thisness and focusing on people…
Mostly, I think identity is a matter of conscious activity, a combination between self-image, reinforcing experience, memory. I do not think identity requires continuity of form or thought (i.e., I can be “me” again after sleeping or being comatose, etc). I do think identity follows the arrow of time backwards, but not forwards.
This last one is the result of some thought experiments around teleportation and duplication devices. If I made a duplicate of myself, is it me or just another person that identifies as me? In this sense, both of us may have a single being as a part of our previous identity (the original) but neither is identified as the other (how would we select between them?), so each has its own identity. So we may look back to lay claim to a part of our identity, but we may not start from the past and move forward necessarily. So nothing forbids two people with identity to lay claim to an identical prior entity, but nothing from the past may lay claim to any posterior entity.
Which, I think, makes sense given that memory is a component of identity.
Now, as for the rest of the OP
Firstly, it is not a mere coincidence that we use the same word in many contexts, but it would be a mistake to suggest that the word must mean the same thing in all contexts. So, absent a context, I do not think there is a "man"ness, "woman"ness, "dog"ness, or any such quality in an abstract, transcendent sense (reference: Platonic forms, certain kinds of transcendental idealism (non-Kantian, non-phenomenological)). However, I would suggest that the context contains, analytically, the meaning of a word to the extent that if I understand a context, and I understand how a word is used in other contexts, it is, in principle, possible for me to understand what the word is supposed to mean in this context (by analogy, similarity, contrasts, etc). For example, a metaphysical idealist believes all of reality is mind-correlative, which contrasts realism, which believes there is a mind-independent reality. However, another use of “idealist” is someone who is often unconcerned with practical matters, where a “realist” (contrasted again) is someone who is concerned with practical matters and various levels of pragmatism. Two contexts, same symbol. Again: it is no accident that we use the same words here, but it would be a huge mistake to suggest they mean the same thing!
So descriptors, to be meaningful, must have a context accounted for. Within a context, a descriptor will sometimes provide a means for its own evaluation (“an idealist believes all reality is mind-correlative; this person does not believe all reality is mind-correlative; this person is not an idealist”). Other times this won’t be the case (what distinguishes a “guiding light” from any other light to a lost traveller?).
This latter question treads on the grounds of the philosophical problem of universals. Depending on who you ask, there are traditionally two answers to “Do universals exist?”
- Yes, they do (realist, not to be confused with either of the two “realist” positions described above (isn’t philosophy fun?))
- No, they don’t exist at all, it only seems like they do (nominalist).
Those who choose the first path are further divided by answering the question “How do they exist?” and those who choose the second path are further divided by answering the question “Why does it seem like they do?”. Some consider the matter closed with conceptualism but I wouldn’t say there’s universal (cough) accord on the matter.
A big post, lots of words, but reading up on some of the terms might get some people acquainted with various positions.