That makes no sense to me. Is there some obligation that all hobbies be useless? One of my hobbies is contributing to open-source software. That's useful, right?

As my fiance is so fond of telling me when I say "I'm off to do some work!" when going to work on my programming side projects (which includes open source development with plans for future closed source commercial projects as well), she replies "Until it makes us any money, it's just a hobby. Have fun."

Ah, good times.

Ask her if she thinks that way about your marriage.

Marriage makes you money (via economies of scale and tax breaks).

She worked and paid our rent and grocery bills while I finished up my CS degree. She can call it whatever she wants. :-)

A => B /= ~B => ~A

Sorry, maybe I'm misreading your notation, but are you saying that modus tollens isn't a valid argument? http://en.wikipedia.org/wiki/Modus_tollens

I think he's using a logical implication notation "|=". as in: A => B logically implies that: ~B => ~A

He just decided to use "/=" rather than "|="...so it looks weird...

Ah, I'd read that as "does not equal", rather than "implies". Thanks pilgrim689.

Actually, if the positive is true, then the contrapositive is also true: http://en.wikipedia.org/wiki/Contrapositive

Ah, I screwed it up :(. I meant to say A => B /= ~A => ~B to convey the fact that if someone says that all useless things are hobbies, then it doesn't follow that things that aren't useless cannot be hobbies, i.e.: that all hobbies must be useless. But as groovy2shoes and MattyDub pointed out, I did it wrongly. Thanks for the kind interpretation pilgrim689 (and indeed, I could have stated that to make the same point), but I was just wrong.