As I mentioned previously, I've been reading the letters of both Kingsley Amis and Philip Larkin. One thing you notice in any Collected Letters book (try Evelyn Waugh's) is old age creeping up on the writers. It's less noticable in Larkin's case; his personality famously made him sound about 70 years old for decades. But with Amis, it's clear that you're reading the thoughts of a young, a middle-aged, and then an old man. This process is chronicled from just outside in his son Martin's book Experience

Don't get me wrong - Amis's letters are wonderful, even the late ones. But a sort of hardening of the personality takes place, kin to atherosclerosis, and it's a common thing to see. What I wonder is how much is due to just plain experience and weariness with the world (seeing the same mistakes being made the same ways, again and again,) and how much has a neurologic base.

Circulatory problems, Alzheimer's (we won't get into the debate about what causes it,) any number of other biological causes affect the number and activity of the neurons. And that, in turn affects higher functions of thought and personality. But that statement broad-jumps over a huge pit of unknown detail. These questions are going to keep everyone in the biomedical sciences busy for a long time, and won't the world be an odd place when answers start to show up?

More on this later. There are many things coming much sooner from modern neuroscience, and we'll have our hands full with those just the same.