From Andrew Gelman:
Work life doesn't have to have a sense of purpose--lots of people work 9-5 and live for the weekends. But being a professor is supposed to be more than just a paycheck. Not many people would go to the trouble of writing a Ph.D. thesis in sociological theory just for the purpose of getting a cushy job.
What happened to Rubinstein is something that I've seen happen to a lot of other people with Ph.D.'s: at some point they get a feeling that their research has no real point and they can't really motivate themselves to go further. That's fine. The question is what do do next. Here are some options I've seen:
1. Keep on doing your research.
2. Shift to a different area.
3. Throw yourself into classroom teaching.
4. Write textbooks.
6. Get involved in education in other ways. Or
become a blogger!
[t]eaching, within its defined parameters, is often its own reward.