The thought experiment asks you to consider a machine that could give you unlimited and diverse kinds of physical pleasures for the duration of your natural life. However, once you choose to “plug in” to the machine, you can never be disconnected. The question Nozick poses is, if such a machine existed, would you plug in?
Nozick thinks our hesitation at plugging into the machine for the duration of our lives highlights an awareness that while pleasure is a good, it is not the only good, nor indeed is it the chief good humans can experience.
In his essay, On Utilitarianism, J.S. Mill asserts the same intuition when he claims that anyone familiar with both physical and mental pleasures will prefer the latter due to their greater quality even if their quantity may be more difficult to achieve.