The same is true throughout the body, of course. If you don't use your muscles, they atrophy. It takes regular activity - intellectual and physical - to maintain the things you have worked to achieve.
I had a great lesson in using it or losing it on Thursday night, when I attended a talk given by a 95-year-old philosophy professor. I hope I live to be 95. And if I make it to such a grand age, I hope that my mental and physical condition allows me to cross the Atlantic, hike up six flights of stairs, and deliver an hour-long lecture in a language other than my native tongue. It was a remarkable thing to watch.
Perhaps my favourite part came during the question period. A man asked a question that the speaker didn't fully understand (at 95, he was rather hard of hearing). As the man was busy smirking at the audience, the professor (quite rightly) told him off for not listening to his response - "it seems this is a monologue, not a dialogue", he said.
I can't say I came out of the lecture with any improved understanding of the mind-body problem, but I was rather inspired to start taking precautions to prolong my own intellectual and physical life. I biked home, even though it was raining, because biking is excellent exercise and the U-bahn is full of germs. Then I had a carrot with my dinner. Drastic measures, I know.
Next weekend I may even go find myself a bike helmet. Or would that be excessive?