Can't = No Ability Whatsoever

Over the course of my recovery, I've met many people who claim to have "big balls".  A person like this will often say, "If I CAN'T do something, I just try harder."  Saying you "can't" do something means you have no physical ability to accomplish the said task.  Humans "can't" fly.  We don't have have the physical ability to fly, because our bodies aren't meant for flight.  When I face a very challenging task, I just think, if I have a hard time doing this, I'll just try harder.