Well, presumably when you mentally masturbate, your thinking is not productive or practical. Just as certain prudish people would have us abstain from the unproductive ways of onanism, those who would censor "mental masturbation" believe thoughts should make something. Like semen, thoughts should be put to use.
To me, thinking is a practice in and of itself and hence is inherently practical. Thinking is a kind of doing such as, say, running. It's an activity. Unless we say that running is physical masturbation as it's not practical. After all, you're not running to get anywhere such as the book store.
The runner might retort that his running makes him more physically fit which, in turn, makes him happier and healthier. In that sense, running is a practical activity.
But couldn't I say the same thing about thinking? Maybe I'm not trying to solve a problem but I'm making myself smarter and therefore healthier and happier. Oh, and the endorphins! Good mental masturbation is a great natural high.
And then there's the fact that thinking makes connections between things and, in so doing, creates the world. So when people are sitting quietly thinking on their own and not trying to solve a problem per se — when they are mentally friggin' themselves — they are making novel connections in the universe, creating new possibilities of life. And what, I ask, is more productive than that?
For example, when I was teaching, I'd sometimes find myself following a peculiar line of thought that had occurred to me mid-lecture. I could tell at some point that my students were not following me. It's not because what I was thinking was so startlingly smart. It was usually because my thinking had turned rather idiosyncratic — it was a train of thinking of my own making, decipherable to no one else, more or less. But this line of thinking would have an odd kind of allure, seducing me, enticing me: Come hither, it'd whisper in a throaty whisper, and I would.
Now, this is often a supremely pleasurable thing to do: to follow an idea into strange territory, making bold — if, at times, stupid — leaps of logic. But talking about it to others quickly becomes not just strange, annoying, and pedantic. It becomes obscene. Ergo, mental masturbation.
In this case, the crime of mental masturbation is akin to the crime of so-called TMI — divulging too much personal information in a social setting. It's a matter of etiquette. But continuing to call it mental masturbation is anti-intellectualism, the ploy of the dim witted. And it gives one of my favorite activities a bad name.
So I'm taking back mental masturbation from the anti-intellectuals. I think alone and for pleasure, dammit, and I'm proud!