Imagine you had a basket full of apples and you wanted to distinguish between the ones that are good and the ones that are bad. You decide to build an apple-sorting machine that would do exactly that. This machine is supposed to sort the good apples from the bad apples. But how can you build this machine without already knowing how to differentiate good apples from bad ones? You can’t. But you also can’t sort the good apples from the bad ones without the machine. So you are left in a position of...