Introduction
Brain surgery is getting better all the time. Though we can't do brain transplants yet, one day we may well be able to. Let's imagine that we are living in a time when they are possible.
Unlike other transplants, of course, the person who survives the operation is presumably the owner of the organ, not the owner of the body! But like all organ transplants, brain transplants involve an intermediate stage. For a while, a brain has to be stored outside its old body before it is connected into a new one. Now suppose that someone—call him Albert—is very badly injured in an accident. His body is hopelessly damaged. Fortunately, his brain was protected by a helmet, and it is unhurt. So a neurosurgeon sets about removing Albert's brain from his body in order to transplant it to a new one. Let's call this surgeon Marie. Marie carefully removes, along with the brain, botha) the sensory nerves that used to carry information from Albert's eyes, ears, nose, mouth and so on, about the looks, sounds, smells and tastes and the feel of the world around him; and
b) the motor nerves that used to carry messages from the brain to the muscles, “telling them” what to do.
Unfortunately, there isn't a spare body available just yet. So Marie puts Albert's brain into a vat of fluid and connects up the main blood vessels to a supply of blood. This is science fiction, so let's add interest by supposing that Marie is a mite unscrupulous. She's willing to do pretty much anything in the interest of knowledge. Here's a spare brain, and she just can't resist investigating it while she waits for a body. So she connects up the sensory nerve endings to an elaborate computer.
The computer is designed to feed those nerve endings with electrical stimuli that are just like the stimuli that Albert got when his brain was properly connected to his body. Thus, when Albert's brain recovers consciousness, the computer feeds it electrical stimuli that produce in the nerves of his eyes the very same electrical signals that used to make him think he was looking around a room. If Marie connected the motor nerve endings to the computer too, she could tell what the brain was trying to do, and the computer could fake the experiences that the brain would have had in a body if it had succeeded in what it was trying to do.Now here's a question. Is there any way Albert could tell that he was being fooled? Most people would say that the answer is no. But if Albert couldn't tell in that situation, then if you were in a similar situation, you couldn't tell either. So what makes you so sure you aren't being fooled right now? Maybe you're part of the first experimental program that will eventually lead to regular brain transplants. The researchers know that you would be very distressed to discover that you had lost your body, so they've deliberately wiped out all memories of the accident. They've faked your experience of reading this chapter in order to start you thinking about the idea of a new body! Later on, maybe, they'll tell you the truth about Marie and her computer, but for now you are “living” like Albert. Of course, if you are being fooled now, then all the things you think are going on around you are not happening at all. This book you think you are reading, for example, is just an illusion produced by a device like Marie's computer. (This is a favorite topic of science fiction in films such as The Matrix.)
Philosophers are often caricatured as being worried about things that it is absurd to worry about. We are supposed to ask questions like “How do I know that the book in front of me is really there?” Without a context, that really can seem a pointless question.
But once we place the question in the context of this science fiction possibility, it does not seem so obviously pointless. Maybe, one day not too far from now, people will find themselves asking this question in all seriousness. Once again a piece of science fiction has led us straight to the heart of a philosophical problem. How do we know about the existence of physical objects? Our maternal robot, M, raised the question of how we know that other people have minds. Now we have to ask an even more disturbing question: How do we know that other people have bodies? Indeed, how do we know that anything exists at all?Questions like these, about the nature of knowledge, belong to epistemology—the philosophical examination of the nature of knowledge. And one way to set about answering the sorts of questions raised by this story is to start by asking what we mean by “knowledge.” If we can answer that question, we'll be in a better position to discover whether—and if so, how—we know that we aren't just brains in fluid, the playthings of an unscrupulous scientist.
2.2