The article poses a hypothetical dilemma:
Suppose there were an experience machine that would give you any experience you desired. Super-duper neuropsychologists could stimulate your brain so that you would think and feel you were writing a great novel, or making a friend, or reading an interesting book. All the time you would be floating in a tank, with electrodes attached to your brain. Should you plug into this machine for life, preprogramming your life experiences? [...] Of course, while in the tank you won’t know that you’re there; you’ll think that it’s all actually happening [...] Would you plug in?. (Anarchy, State, and Utopia, p. 3)Some people would gladly plug in the machine; others would not. For those in the latter group, why not? Don't we aim to be happy?
The writer's answer is that happiness is like knowledge. That is, it is possible to think that you're happy and be wrong about it. For example, if you think you have great friends, but they actually hate you (or if they're actually a set of fine silverware and china), you might think you're happy but you're not.
One thing I don't particularly like about this idea is that the way he defines "happiness" does not really match the way it is usually used. Happiness is usually thought of as an experience, not as a relationship between experience and external facts. But the writer refers to this concept as "pleasure" instead. Happiness is to knowledge as pleasure is to belief. The idea is that we prefer happiness over pleasure, just as we prefer knowledge over belief.
I don't have anything else to add, I just thought it was interesting.