"Going over it in my head doesn't count"

At the lights you didn't go back for the straighteners. Instead you replayed the moment. The bathroom, the plug, your hand on the switch. You ran it three or four times, trying to catch a detail that would settle it. That felt like a reasonable compromise. You weren't checking. You were just thinking.

Radomsky and Gillian Alcolado tested exactly this. Sixty-two students checked a stove either physically or in their heads, repeatedly. Mental checking meant going over the previous check in imagination, the way you did at the lights. The result was that repeated mental checking reduced memory confidence, vividness and detail for the mental checks, just as physical checking did for physical ones. There was also a slight drop in accuracy in each case, which the physical-only studies hadn't found. Going over it in your head is checking. It fades the picture the same way.

This matters because the replay is the recheck that goes everywhere with you. You can leave the door behind. You can't leave your head behind, and so a person who's stopped walking back to the door can still spend the drive to work checking it, and arrive as unsure as if they'd gone back four times. The replay also has no natural end, because there's no handle to push. You can run the bathroom scene until you get to work, and each run makes the scene less trustworthy.

I'd add my own observation about why the replay is so tempting. It feels like remembering, and remembering seems harmless. But there's a difference between a memory that surfaces on its own and a memory you go looking for, over and over, with a question attached. The second is a search, and the search is for a detail that will end the doubt, and the searching wears the detail away. By the fourth run you're not remembering this morning. You're constructing a plausible morning from all the mornings, and you know it, and that's the doubt.

So the rule this book will come to has to cover the replay as well as the walk back. Checking once means once in the world and none in the head. That's harder, and chapter 18 is about how.