> This idea \[that modal operators perform quantification without making use of explicit variables and binding], when expressed mathematically, has turned out to be the most significant milestone in the history of modal logic. ## References * Blackburn, van Benthem, Wolter, Handbook of modal logic, Elsevier, 2007 * Carnielli, Pizzi, Modalities and Multimodalities, Springer, 2008 ## Contributors * Yde Venema, [website](http://staff.science.uva.nl/~yde/): algebraic- and coalgebraic aspects of modal logic. e.g §6 in the "handbook". * Valentin Goranko and Martin Otto. "handbook" §5 Model theory of modal logic