Many old wisdoms are outdated or misinterpreted regularly, but I don't think "you can afford to lose" is one of them. Most people understand what it means without trying to be philosophical about it. I'd be surprised if someone read that, understood the examples that people use to explain it, and arrived at the conclusion that in gambling, you every money need to turn into profit or something similar. That's the goal for sure, but that shouldn't be the takeaway. I usually tell someone don't use 80% of your monthly salary for trading, I'm sure it applies here too.