Among the seven deadly sins, none is more interesting to study for its historical development and the complexity of its meaning than the sin of acedia. For while its companions — with the possible exception of avarice — remained fairly static through the centuries of medieval thought, and indeed have remained so to the present, acedia presented a variety of faces and changed in its very nature, from the moment when it entered Christian teaching in the West to the fifteenth century. In Christian thinking, pride has always been pride; its psychological roots may have been explained in agreement with different philosophical and psychological systems, and its manifestations may vary according to changing attitudes and experiences, but its nature has remained essentially unchanged. The same is true of envy, wrath, lust, and the others. But not so with acedia. A reflection of the complexity which this concept acquired during its medieval lifetime can still be seen in the totally different connotations which its names have for the cognoscenti of today. The ancient term acedia fascinates because it suggests such phenomena as spiritual dryness, ennui or WeItschmerz, while its vernacular equivalent sloth, as everyone readily agrees, stands for something so common and ordinary that it hardly deserves a second thought.