Professor Daniel N. Robinson suggests that any student of philosophy - at least that idea that philosophy is meant to be used in guiding one's own life - answer this question:
Suppose you could be given any life of your choosing played out into you by way of stimulating the brain. What are the grounds on which you would not choose this? What is the basis on which you would choose a life that is frustrating, in which desires remain unrequited, in which you could not achieve a sought level of excellence, over a life in which, by way of the direct stimulation of the brain, all of these desiderata would be achieved, and all negative ones eliminated?
This is an account of what is also known as Robert Nozick's "Experience Machine" question. Nozick brings up this question against hedonism - against any theory, in fact, that says that all we are after in life is some brain state, be it happiness or pleasure or contentment. It is raised against the idea that the only thing we seek is to have the molecules in our brain arranged in a particular way and its currents travelling in a particular fashion and, of all of the things in the world, this is the only thing that matters.
The objection states that, if this is the case, then it seems that reason would demand that we choose this if it were made available. If putting the brain in a vat wherein its molecules could be organized in the appropriate way and its currents could be sent travelling in the proper fashion could be accomplished, then the person who claims that this is the ultimate goal of all human action should see no reason not to jump at this option.
If one would reject this, then . . . why?
Well, I would reject this kind of life. I contemplate the offer and it is of no interest to me. I could, perhaps, be persuaded to enter the experience machine for short periods of time to have experiences that I would enjoy - as a form of entertainment. However, the time would come where I would have to set entertainment aside and do the things that I want to do - accomplish the things I want to accomplish. This means actually doing them, and actually accomplishing them. Or, at least, it means actually trying to do so, even if I should fail.
I have a desire - a goal that I selected for myself in high school - to leave the world a better place than it would have otherwise been. I think that, in my years of study and experience, I have learned some things that it would be useful to others to know. I wish to contribute what I have learned to the total sum of human understanding, in the hopes that at least some of it is actually useful.
A desire is a propositional attitude. That is, to say, any desire that any person has can be expressed as an attitude (desire) towards a proposition P. I desire that I make the world a better place than it would have otherwise been.
The attitude that desire attaches to a proposition is a motivational interest in realizing that proposition - in making the proposition true.
The problem with Nozick's experience machine is, quite simply, that it cannot make these propositions true. The experience machine can feed me the sensations that might cause me to believe that I have made the world a better place. But, in truth, the world is no better off. In fact, the world itself is unchanged. And that makes all the difference.
This does not mean that "leaving the world a better place than it would have otherwise been" has some sort of intrinsic value. It does not imply that such a state has value, in particular, that is independent of the fact that I desire it. It is still in virtue of the fact that I desire to leave the world a better place than it would have otherwise been that this has value. However, it is also the case that in virture of my desire what has value (to me) is that "I leave the world a better place than it would have otherwise been" be made true. Since this is what has value, and life inside the experience machine cannot provide this, then life inside the experience machine does not have value.
This might not be true of anybody. I have admitted to having some desires that can be fulfilled within the experience machine - in exactly the same way that I have some desires that can be fulfilled inside of a movie theater. There may be some people for whom all, or nearly all, of their desires are desires of this type. Such a person may choose to live within the experience machine.
However, it is important to note, that the difference between us is not that I have desires that I seek to realize and they do not. The difference is that I have desires that cannot be realized within an experience machine, and they have desires that can be realized within an experience machine. A person who has a desire "that I experience what it is like to be successful World War II ace" has a desire that can be realized within an experience machine, and thus has reason to enter such a machine. The machine can make the proposition that he is having such an experience true.
All desire, then, is a desire that P, which motivates the agent to realize P. The difference between those who would enter into such a machine and those who would not is found in whether P is a proposition that can be realized within the machine, or a proposition that cannot.
For me, a great many of those propositions cannot be realized within the machine. That is why I would choose not to enter - not for more than a couple of hours at least.