As always, the argument is over originality. Who comes first- which subjectivity is objective? The answer they will come to is its own, just as each is their own. (One could think- a cell's daughters are both daughters? But who accepts the word of a microbe, anyway...)
They continue on, until finally- it's easy for them to reverse divisions. They touch-
And that's how the first and second goos to invent a time machine became the third.