Could you determine purely by introspection on your thought processes, that the charges of your neurons' channels were carried by potassium ions rather than chloride? Could a computer program figure out through introspection, without any webcam or robotic hands or pretrained facts, whether it was implemented on top of a mechanical abacus or vacuum tubes or transistors? No, obviously, in the case of the computer, because all the numbers would be the same whatever implements them. And you also can't tell from the inside what materials you are made of. You cannot by deep introspective perception detect a proof that your brain must run on potassium ions rather than chloride; this is among the reasons the ancient Greeks did not figure that out. These same thoughts you are thinking could be implemented on top of different biological materials organized into the same patterns; so you can't differentiate between possible biological details from staring at your own thoughts that could be the same either way. All you can detect from the inside is the pattern in the material, not the material making up the pattern. And so too with it being impossible for you to determine by introspection alone -- without cracking open a skull to look, without reasoning from what other people have merely told you about how the world works -- whether you are made inside of ions rushing into cell bodies to transmit depolarization across myelinated fibers, or a different sort of electrical charge traversing a different sort of channel. If tomorrow it were revealed that there had been a huge conspiracy to lie to you about whether neurons ran on potassium or chloride, or maybe that they'd really been transmitting a quite different sort of signal all along, you'd have no right to cry, "But I observed with my own sensorium that it was not so! How could I say 'I think therefore I am' if it were chloride? How could the blueness of a sunny day's sky have the exact quality that it does, if it wasn't actually potassium?" All you know about the materials making up your brain is what other people have claimed to you. You have never felt for yourself that it was potassium ions. You cannot see, by looking inside your mind, which particular materials have been arranged into you. It is not an element of who you are. You have introspective access to the fact of your own consciousness. You cannot in principle distinguish by introspection which exact materials you are made out of. So it can't be about the potassium ions specifically, and there you have it.