This is
something you can say to
anyone about
anything. It's especially useful when you're losing an
argument. The idea is that you can't
prove anything because you have no way of knowing if your
senses are even
correct (yes, like
The Matrix). Hopefully the
person you're arguing with will
forget about what you were just arguing about and think about what you just said. The
problem with it is that it makes any argument
impossible, so it gets
annoying very
quickly.
There's no need to softlink to one of those "your radical ideas have already occured to other people"-type nodes, I'm already aware of that.