Physically, you’re always changing. Your body is never the same. Your blood is always flowing. Your heart is always pumping. Cells die and new cells are born all the time. And living cells are changing all the time: their biochemical processes only stop when the cell dies.
Nor is your mind ever still. New thoughts, ideas, imaginings are constantly emerging then fading away. And if you think you don’t always think, think again: you’re constantly receiving information about the “outside world” via your senses. Even when you’re asleep you’re monitoring internal processes such as breathing as well as external factors such as temperature. Your brain is a perpetual motion machine – neurons are firing all the time, in sleep and in wakefulness, even in coma.
So, if you are always changing, if you are never the same from instant to instant, then what exactly is it that corresponds to you: which set of cells, which thoughts, which feelings, which sense perceptions, which electrical impulses? Is there anything that stays the same for long enough to warrant being a something? Is the current “me” (whoever that is) responsible for the actions of a previous “me” (whoever that was)?
It’s difficult if not impossible to come up with a clear and trustworthy definition of the thing we label as the “self”. The self is a slippery beast that wriggles and squirms out of the grasp of any mind. There's the issue of where the self lives. There's the issue of whether the “I” that I am now is the same as the “I” of ten years ago, or a minute ago, or even the “I” in the memory or mind of another.
And then there's the issue of what I believe and what I perceive. The experiments of Benjamin Libet highlight the problems of identifying who or what possesses the contents of consciousness, who or what if anything is the agent capable of intentionality, and whether “free will” actually means anything at all.
Light travels very quickly (186,000 miles per second) but it still takes time for a photon to get from A to B. And it takes time for data about the “outside world” to be received and processed by the mind. Everything I see, hear, smell or feel is in the past. By the time I become aware of what I am looking at, it is no longer the same thing as it was before. When I look through a telescope at, say, the nearest star (Proxima Centauri), I see Proxima Centari as it was 4.24 years ago. (Proxima Centauri is 4.24 light-years away from our sun, Sol.) The data on which a Self constructs reality, constructs itself, is old data. Not only is it old, it is unreliable data, pertaining to that which no longer exists.
Some believe that right now is the only time there is. Now is the eternal instant when the three times are one, as the Tibetan Book of the Dead puts it* (the three times being past, present and future). Tibetan Buddhists (among others) believe that the eternal moment is all there is, was and ever will be; that all I am, have, do and know is the whole of the eternal moment, per the "Tat Tvam Asi" of non-dualism, in which the soul/self/consciousness is the only reality (whatever that means).
I don’t know about that; in fact I don’t truly know anything at all.
* Tibetan Book of the Dead, Penguin Classics Deluxe Edition, published in Britain 2005, p. 47