[has2k1]

May be strange at a first glance but binary/boolean logic/reasoning/quantification is just a special case of probability theory. This is covered exhaustively in Probability Theory: The Logic Science, by E.T Jaynes.

And so "tend" and "iron law" are interpreted as assertions of different levels of certainty, where "tend" implies a certainty greater than 50% and "iron law" a certainty closer to 100%.