There's a widely-held picture of human motivation that makes it difficult to see this. That picture goes like this. Ultimately, it says, everyone always acts for selfish motives. Whenever we do something on purpose, it's our own purpose that we're trying to achieve. We're always pursuing our own ends, and trying to satisfy our own desires. All that any of us are really after in life is getting more pleasant experiences for himself, and avoiding painful ones. Sometimes it may seem that we're doing things for other people's sake. For instance, we give money to charity, we buy presents for our children, we make sacrifices to please our spouses. But if you look closer, you'll see that even in cases like these, we're still always acting for selfish motives. We only do such things because it makes us feel good and noble to do them, and we like feeling noble. Or we do them because when people we care about are happy, that makes us happy too, and ultimately what we're after is that happiness for ourselves. Hence, since the only aim we have in life is just to have pleasant experiences, Nozick's experience machine gives us everything we want, and it would be foolish not to plug into it.
Now, I grant that some people may be as selfish as this picture says. But I doubt that many people are. The picture rests on two confusions, and once we clear those confusions up, I think there's no longer reason to believe that the only thing that any of us ever aims for in life is to have pleasant experiences.
The first confusion is to equate "pursuing our own ends, and trying to satisfy our own desires" with "acting for a selfish motive." To call a motive or aim "selfish" isn't just to say that it's a motive or aim that I have. It says more than that. It says something about the kind of motive it is. If my motive is to make me better off, then my motive is a selfish one. If my motive is to make you better off, then my motive is not selfish. From the mere fact that I'm pursuing one of my motives, it doesn't follow that my motive is of the first sort, rather than the second.
Ah, you'll say, but if my aim is to make you better off, then when I achieve that aim, I'll feel good. And this good feeling is really what I'll have been trying to obtain all along.
This is the second confusion. It's true that often when we get what we want (though sadly not always), we feel good. It's easy to make the mistake of thinking that what we really wanted was that good feeling. But let's think about this a bit harder. Why should making someone else better off give me a good feeling? And how do I know that it will have that effect?
Consider two stories. In story A, you go to visit the Oracle, and in her waiting room you see a boy bending spoons and a girl levitating blocks. You feel this inexplicable and unpleasant itch. Someone suggests as a hypothesis that the itch would go away if you gave the girl a spoon too. So you do so, and your itch goes away.
In story B, you walk into the same room, and you don't like the fact that the girl has no spoon. You would like her to have a spoon too. So you take a spoon and give it to the girl, and you feel pleased with the result.
In story A, your aim was to make yourself feel better, and giving the spoon to the girl was just a means to that end. It took experience and guesswork to figure out what would make you feel better in that way. In story B, on the other hand, no guesswork or experience seemed to be necessary. Here you were in a position to straightforwardly predict what would bring you pleasure. You could predict that because you had an aim other than making yourself feel better, you knew what that aim was, and usually you feel pleased when you get what you want. Your aim was to give a spoon to the girl. Your feeling of pleasure was a consequence or side-effect of achieving that aim. The pleasure is not what you were primarily aiming at; rather, it came about because you achieved what you were primarily aiming at. Don't mistake what you're aiming at with what happens as a result of your getting what you're aiming at.
Most often, when we do things to make other people better off, we're in a situation like the one in story B. Our pleasure isn't some unexplained effect of our actions, and what we're primarily trying to achieve. Our pleasure comes about because we got what we were primarily trying to achieve; and this makes it understandable why it should come about when it does.
Once we're straight about this, I think there's no argument left that the only thing anyone ever aims for in life is to have pleasant experiences. Some people do aim for that, some of the time. But many cases of giving to charity, making sacrifices for one's spouse, and so on, are not done for the pleasure they bring to oneself. There's something else that one is after, and pleasure is just a pleasant side-effect that sometimes comes along with getting the other things one is after.
Nozick said that most of us do value more than our own experiences, that there are things that we value that we'd miss out on if we plugged into the Matrix. I think Nozick is right. He's right about me, and he's probably right about you, too. We can easily find out. I've devised a little thought-experiment as a test.
Suppose I demonstrate to you that your friends and I are very good at keeping secrets. For instance, one day when Trinity isn't around, we all make lots of fun of her. We read her journal out loud and laugh really hard. We do ridiculous impersonations of her. And so on. It's hilarious. But of course we only do this behind Trinity's back. When she shows up, nobody giggles or snickers or anything like that. You're completely confident that we'll be able to keep our ridicule a secret from Trinity. She'll never know about it.
Suppose I also demonstrate to you that I am a powerful hypnotist. I can make people forget things, and once forgotten they never remember them. You're convinced that I have this power.
Now that you know all of that, I offer you a choice. Option 1 is I deposit $10 in your bank account, but then your friends and I will make fun of you behind your back, the way we made fun of Trinity. If you choose this option, then I will immediately use my hypnotic powers to make you forget about making the choice, being teased, and all that. From your point of view, it will seem that the bank made an error and now you have $10 more in your account than you had before. So in terms of what experiences you will have, this option has no downside. You won't even have to suffer from the expectation of being secretly teased, because I'll make you forget the whole arrangement as soon as you make your choice.
Option 2 is we keep things as they are. I pay you nothing, and your friends are no more or less likely to make fun of you behind your back than they were before.
So which would you choose?
When I offer my students this choice, I find that at least 95% of them choose Option 2. They think that the teasing would be a bad thing, even though they'd never know it was going on.
If the teasing doesn't seem so bad, then change the example. Say that in Option 1, your lover is cheating on you, but you never know about it. Or say that we're torturing your mother, but you never know about it. In every version, your experiences are smooth and untroubled, plus you get a little extra money. Which option would you choose?
If you find Option 2 more attractive, then that's support for Nozick's claim. The experience machine wouldn't give you everything you value. Option 1 gives you no experiences of being teased. It gives you no evidence that your lover is cheating on you, or that your mother is being tortured. But you don't just want to have experiences of things going well for yourself and your mother. You value really not being teased, really having a faithful lover, and really having an untortured mother.