In the 1966 film Fantastic Voyage, starring Raquel Welch, humans are shrunk down to the size of body cells and injected into another (full-size) human being. Actually, it was a little spaceship of sorts that was injected, but Raquel was inside it. I can't remember precisely what the reason for this injection was, but, as I recall, some sort of errand of mercy was involved. So, let us engage in what philosophers call a thought-experiment. You are Raquel Welch aboard your little spaceship. But this ship has been modified. Instead of having to be injected into the blood supply, it now has a boring or tunnelling device, and you can bore directly into the person's body. The whole errand of mercy scenario would no doubt have to be changed given this technological innovation – we now seem to have a sort of inverse Alien scenario, and our film should now be directed by Ridley Scott – but we needn't worry about that. Our concern is not ethics but philosophy of mind.

Your preferred port of entry, let us suppose, is the skull. So, bore away you do. First you make your way through the skin: the boundary between your experimental subject/victim and the outer world. That's easy. The next layer, the skull, however, proves a lot more difficult, and you spend quite some time boring your way through that. Eventually you break through into the grey, gooey mess that is the brain. Actually, at your new cellular size, it may not appear grey and gooey at all.