For those of us alive on that day, time was split in half. There were the years that came before terrorists flew airplanes into the World Trade Center and Pentagon — and those that came afterward.

We are a country surrounded by oceans, cosseted by space, a geographical feature I’m most aware of since our return from Europe the day before yesterday. Our cities were not bombed. We had no relatives forced to surrender or fight for the resistance. Our relative isolation gave us an air of invincibility that was punctured that early autumn Tuesday 24 years ago.

Since then, a generation has passed away and another has been born. The good bots at Google inform me that approximately 60 percent of the world’s population alive on September 11, 2001 is still alive today. Which means, of course, that approximately 40 percent is not. Those of us who remember scarcely outnumber those who do not.

I recall saying to my kids shortly afterward, “Life will never be the same.” For me, and for many, it hasn’t been. But for them — and even more so for those born after the terrorist attacks — the post 9/11 world is the world they inhabit. They live in its wake.