It's not a great example, and I'm sorry for it. Let me try another one. In the study of eukaryotic cell cycle, there's a notion of a checkpoint. For example, if

It's not a great example, and I'm sorry for it. Let me try another one. In the study of eukaryotic cell cycle, there's a notion of a checkpoint. For example, if you damage the DNA of a yeast cell severely, it will continue its cell cycle until some point and then arrest instead of arresting directly, and you can find mutants that don't arrest. There's a whole set of mutants that were used to figure out the checkpoints in the cell cycle. Of course there's no little physical switch. This is a giant mass of molecules bouncing around. A fellow I know did some interesting work on replacing simple checkpoint models with entrained oscillators. But this makes the checkpoints look like an abstraction. That's our view of it, though. It's not an abstraction. It's something that looks like one.