What does the pronoun ‘I’ refer to?

What does the pronoun ‘I’ refer to? Wittgenstein (Philosophical Remarks, §64).

‘I have a pain’ is a sign of a completely different kind when I am using the proposition, from what it is to me on the lips of another; the reason being that it is senseless, as far as I’m concerned, on the lips of another until I know through which mouth it was expressed. The propositional sign in this case doesn’t consist in the sound alone, but in the fact that the sound came out of this mouth. Whereas in the case in which I say or think it, the sign is the sound itself.