“Experience has shown that unless some simplifications can be made, the amount of computation involved increases at least as fast as the square number of equations. This we call the Square Law of Computation.” [source]
Whenever I see a n2 expression I dig. O(n2) seems to be a fine rough metric for a lot of things. And the above was first stated in 1975, almost five years before Metcalfe. Which reinforces my admiration to Metcalfe, for he was the first to link the complexity of a problem with value in such a back-of-the-envelope metric.