aka The Fuzzy Pattern Theory of Identity
Identity is not usefully defined as a single point in thingspace. An "I" which only exists for an instant (i.e. zero continuity of identity) does not even remotely correspond to what we're trying to express by the word "I" in general use, and refers instead to a single snapshot. Consider the choice between putting yourself in stasis for eternity against living normally; a definition of "I" which prefers self-preservation by literally preserving a snapshot of one instant is massively unintuitive and uninformative compared to a definition which leads us to preserve "I" by allowing it to keep living even if that includes change.
Identity is not the current isolated frame.
So if none of those are what "I"/Identity is based on, what is?
Some configurations of matter I would consider to be definitely me, and some definitely not me. Between the two extremes there are plenty of border cases wherever you try to draw a line. As an exercise: five minutes in the past ete, 30 years in the future ete, alternate branch ete brought up by different parents, ete's identical twin, ete with different genetics/body but a mindstate near-identical to current ete, sibling raised in same environment with many shared memories, random human, monkey, mouse, bacteria, rock. With sufficiently advanced technology, it would be possible to change me between those configurations one atom at a time. Without appeals to physical or causal continuity, there's no way to cleanly draw a hard binary line without violating what we mean by "I" in some important way or allowing, at some point, a change vastly below perceptible levels to flip a configuration from "me" to "not-me" all at once.
Or, put another way, identity is not binary, it is fuzzy like everything else in human conceptspace.
It's interesting to note that examining common language use shows that in some sense this is widely known. When someone's changed by an experience or acting in a way unfitting with your model of them it's common to say something along the lines of "he's like a different person" or "she's not acting like herself", and the qualifier!person nomenclature that is becoming a bit more frequent, all hint at different versions of a person having only partially the same identity.
Why do we have a sense of identity?
For something as universal as the feeling of having an identity there's likely to be some evolutionary purpose. Luckily, it's fairly straightforward to see why it would increase fitness. The brain's learning is based on reward/punishment and connecting behaviours which are helpful/harmful to them, which is great for some things but could struggle with long term goals since the reward for making the right/punishment for wrong decision comes very distantly from the choice, so relatively weakly connected and reinforced. Creatures which can easily identify future/past continuations using an "I" concept of their own presence have a ready-built way to handle delayed gratification situations. Evolution needs to connect up "doing this will make "I" concept future be expected to get reward" to some reward in order to encourage the creature to think longer term, rather than specifically connecting each possible long term beneficial reward to each behaviour. Kaj_Sotala's attempt to dissolve subjective expectation and personal identity contains another approach to understanding why we have a sense of identity, as well as many other interesting thoughts.
So what is it?
If you took yourself from right now and changed your entire body into a hippopotamus, or uploaded yourself into a computer, but still retained full memories/consciousness/responses to situations, you would likely consider yourself a more central example of the fuzzy "I" concept than if you made the physically relatively small change of removing your personality and memories. General physical structure is not a core feature of "I", though it is a relatively minor part.
Your "I"/identity is a concept (in the conceptspace/thingspace sense), centred on current you, with configurations of matter being considered more central to the "I" cluster the more similar they are to current you in the ways which current you values.
To give some concrete examples: Most people consider their memories to be very important to them, so any configuration without a similar set of memories is going to be distant. Many people consider some political/social/family group/belief system to be extremely important to them, so an alternate version of themselves in a different group would be considered moderately distant. An Olympic athlete or model may put an unusually large amount of importance on their body, so changes to it would move a configuration away from their idea of self quicker than for most.
This fits very nicely with intuition about changing core beliefs or things you care about (e.g. athlete becomes disabled, large change in personal circumstances) making you in at least some sense a different person, and as far as I can tell does not fall apart/prove useless in similar ways to alternative definitions.
What consequences does this theory have for common issues with identity?
- Moment to moment identity is almost entirely, but not perfectly retained.
- You will wake up as yourself after a night's sleep in a meaningful sense, but not as quite as central example of current-you's "I" as you would after a few seconds.
- The teleporter to Mars does not kill you in the most important sense (unless somehow your location on Earth is a particularly core part of your identity).
- Any high-fidelity clone can be usefully considered to be you, however it originated, until it diverges significantly.
- Cryonics or plastination do present a chance for bringing you back (conditional on information being preserved to reasonable fidelity), especially if you consider your mind rather than your body as core to your identity (so would not consider being an upload a huge change).
- Suggest more in comments!
Why does this matter?
Flawed assumptions and confusion about identity seem to underlie several notable difficulties in decision theory, anthropic issues, and less directly problems understanding what morality is, as I hope to explore in future posts.