⁶Quote from Seneca, Letters to Lucilius, LXXXI.20. Gratitude was also strongly recommended by Epictetus (Discourses, I.6. II.5.10, II.16.28) as well as Plutarch (On the Tranquillity of the Mind, 14). Recently, it has become a topic of great interest in positive psychology (Emmons and Shelton, 2002; Wood et al., 2010; Watkins, 2013); appreciation is a related construct that may be defined as something more general (Fagley, 2016).