 The following paragraph collects the background technical notions needed to interpret the modal languages used in this entry. The reader already acquainted with modal logic can skip it. For a more in depth exploration one can consult the related entry on  modal logic  (Garson 2014). Well-known classic textbooks are Modal Logic: An Introduction (Chellas 1980), which focuses on non-normal modal logics, and Modal Logic (Blackburn et al. 2001), which focuses instead on a more mathematical treatment of normal modal  logics.[2]