About where to draw the line:
That "line" is a simplification tool humans created to apply to sundry topics. Unfortunately it often fails to be true to what we think when we draw it. There's a field called "fuzzy logic" that gives truth values of 0, 1, and every number in between (well, no tensors or complex numbers ;) anyhoo, let's say we drew the line at the boundary of your internal network (your router/uplink/whatever). Ok, great, you can do whatever you want on your side of the network. As soon as you draw that "line", crackers are going to start exploiting that constant. Just like heads of state who are security conscious don't travel the same route every day, varying you network security has benefits...
Um, basically i'm just writing this because I'm against the "line" idea itself... I feel it's a dumming down, which sometimes might be useful or even necesary, but not in this case.
P: this assertion is not provable.
It is provable that an assertion equivalent to P exists.