How much can I afford to lose?

Faceless Bureaucrat:  A word of advice from my Uncle when I share my plans: "Never trade more than you can afford to lose".  Apparently this is Rule #1 of Trading.  I'm not really completely sure what that means - I can't afford to lose any money....who can??! Does that mean I can't trade?  I think about this long and hard, as we Faceless Bureaucrats are prone to do.  And then I resolve to Start Small.  If I'm crap then I won't lose much and can put it down to experience.  But I don't like the concept of losing, or the horror of seeing money that could usefully be deployed securing shoes or nice dinners out just disappearing into the ether that is The Market.  I hope the free seminar I'll be attending shortly will provide more reassurance about this whole issue...