Just because some properties of the PhD and the viva are difficult to count, that doesn’t mean they’re infinite.
There’s only so many papers you could have read.
Only so many ideas you’ve had time to consider.
Only so many experiments you could have tried.
Only a few months at most for you to prepare.
Only so many questions that you’ll be asked.
Only so many words you could use to answer a question.
Only a short time really that you’ll be in the viva.
There are real boundaries. You might not be able to see them but they are there.
You can’t do everything, so think about the finite steps you can take to get yourself from where you are to where you want to be.