The human brain is like a microcosm of a perfect utopian world.
Everything has a purpose and everything works in perfect harmony.
If something gets damaged, then another part comes in, learns the job and takes over.
Dementia or Alzheimer is different.
It destroys that world forever.
It's a malign physical invasion.
It starts by taking out small pockets but then it spreads.
It starts to eat away the whole infrastructure.
At first it's as though there are little pockets of rottenness surrounded by normality, but then normality itself is overrun.
In the end there's just rottenness.
But every world is different and every invasion is different, unique to the specific life of the sufferer.
Who he is, and who he was.