I've been wondering recently what is "more than you can afford to lose"?
should there be a distinction between "more than you can afford to lose" and "an amount that you can afford to lose, but it's quite a large percentage of what you own"...?
It's really up to the individual. You can set hard limits at the level of "an amount where if you lost it you'd be bankrupt/in serious trouble" etc, but for each individual what they're comfortable with risking on an investment/speculation, what they can recover, and what they can mentally and emotionally endure are all different for each person.
General principle: don't invest an amount where if you lose it your life will change for the worse in significant ways.