Comment 257 by Zeuglodon

Agreed, which is why I hope my sidenote for you in Comment 253 helps out a little. You mentioned you needed a formal proof, and I noticed a little overlap, if it gives you food for thought. Here, I'll save you the bother of fishing my old comment and copy it here:

I see your point, but I believe that the argument against non-functionalism is even stronger. It's not just that any non-functional parts are beyond the reach of direct evidence, but that we can see that none of the evidence we have need be interpreted as to indicate non-functionalism.

To put this in terms of your 'event horizon' argument, there was for a while a belief in something called 'dark flow', a general drift of galaxy clusters which seemed to indicate that there was some huge mass beyond the visible universe, a huge mass that had earlier on been close enough to have a gravitational influence. However, the evidence (I think) has now faded. What people thought was evidence for the huge mass turns out not to be so. Of course, some huge mass could still exist, but we have no reason to believe it does.

The problem for non-functionalism that it concedes that in terms of reasons for our thoughts, it's possible for us to get a complete understanding that does not include anything non-functional.

This also means that non-functionalists have to accept that we can't be experts about our own thoughts, because to be an expert means to have justified wisdom, and yet no thoughts about non-functionalism can be justified.

We really are into invisible pink unicorn territory!

To be honest, I think non-functionalists are asking the wrong questions. The question of non-causal aspects of mind is very strongly linked to the question of existence in general. They seem to me to be confusing the question of why they have experiences with the question of why the and their thoughts exist.