What am I? In which I mean, what does it mean to be me. Am I my body? In which case, would I cease to be me if my foot were amputated? If my vision starts going, would my glasses be an extension of myself? Perhaps it is not so much by body, but my mind which concerns me. After all, we could theoretically remove or replace almost all of one's body and there would still be the sense of I.
So perhaps I am a mind, "cogito, ergo sum". But what about a mind is me? Is it my thoughts? My experiences? My memories? My mental states? My desires? These change over time, get damaged or degraded with age. As far as I can see, I is the sum total of all these things, a unity that can be reduced but not abandoned. Am I an I without thoughts? No one thought makes me I, but I requires thought. My memories fade, get distorted, or even invented, but I has memories even if those memories aren't necessary for I.
No doubt there's some continuity to the process. Is I the one who had my childhood? Am I the same I as the I who as a teenager had a different outlook on life than I now? Does I become a different I as time goes on?
The I seems an obvious irreducible entity, that can be wholly reduced away. Of course there's an I, it's the most obvious fact in the world and perhaps the only fact about the world we can hold with certainty. But to pin down what I is leaves nothing left, there's nothing we can grasp that is irreducibly I. For I is a unity of many different things that I is composed of.