The acidity function is a thermodynamic quantitative measure of acid strength for non-aqueous and concentrated aqueous Brønsted acids, with acid strength being defined as the extent to which the acid protonates a base of known basicity. The acidity function, which was developed, both theoretically and experimentally, by Louis P. Hammett of Columbia University during the 1930s, has proven useful in the area of physical organic chemistry where it has been used to correlate rates of acid-catalyzed reactions and to quantitate the (...) acidity of superacids, acids with protonating abilities greater than pure sulfuric acid. All Brønsted acids can now be compared using a common measure. Karl Popper’s seminal idea of theory falsification does not apply here because of the many successful applications of the acidity function. Likewise, Thomas Kuhn’s idea of a paradigm shift does not apply here, even though the acidity function concept was revolutionary, because the acidity function is commensurate with classical concepts of acidity. (shrink)