We change from second to second, literally picosecond to picosecond as our fundamental particles bounce around randomly. We are dependent upon the meat we live in (for now), which is easily altered or damaged, and even beyond our physical structure much of what we are as individuals is dependent upon the chemical "soup" within our bodies.
This came to mind recently as I was reading Accelerando, an intriguing novel in which the protagonist relies on technology external to his brain for processing power and feels like a very different person when that technology isn’t available. Humans will certainly become continually more “interfaced” with technology in the coming decades, to the point that human and device will become parts of a whole.
Other sci-fi books that I have read in recent years that address this theme in some way are Altered Carbon and its two subsequent books by Richard K. Morgan, also Peter F. Hamilton's Void series where some of the characters progress from being so called "natural" humans to enhanced humans with nanoscale biological enrichment to ultimately becoming non-corporeal beings in a virtual environment.
I don't have the answer, but it is an interesting question to ponder and perhaps write upon.
