The sign is arbitrary because language, though arising from nature, is not demanded by nature.
Here’s something fun. You can actually do it if you want, but I think reading it’s probably just as effective for getting the idea across:
On a sheet of paper, with a pencil, write anything you want–something short, to keep this manageable; maybe something about how awesome your day’s going to be. Then, just to double down, write it all over again, just the same.
Now, erase the first version. Put the paper aside and go to your computer or phone. In some text field somewhere, type another message to yourself: maybe it’s different, though the possibilities for questioning seem exponentially increased if it’s the same as the message you originally wrote. At any rate, make sure you do it at the very beginning of the text field.
Whatever you’ve chosen to type out, skip a few lines and do it again. Don’t copy and paste, either; go through the motions again, pay attention to the fact that you’re doing what you’re doing.
Now, of course, erase the first version and all the spaces before the second. There should only be one version of the statement on your screen, at the very beginning of the text field.
So, now, what’s on your screen? Is it the ‘Second’ version of the message? If so, why? What, at this point, distinguishes what’s there now from what was there when you typed the message originally? You can clearly see that the two versions of the message you wrote are still two versions, despite the fact that one was ‘erased’ (its space, its impression maintained). And, if you typed the same message that you wrote, how many versions of that message exist now? Is it some different number than once existed? Does the difference matter?