The term nostalgia dates from the end of the seventeenth century when Johann Hofer, a nineteen-year-old Swiss physician, devised it for his doctoral thesis on the causes for the melancholic state of Swiss serving as hired soldiers in the armies of other European powers or working or studying away from their native area. Nostos in Greek means home, the homeland, while agie means longing, yearning for. As its usage became more popular from the second half of the nineteenth century in west-European languages, it gradually entered Russian, Persian, and Arabic, among other languages, even when they already had words expressing homesickness, in other words spatial nostalgia, and temporal nostalgia, a nostalgia for the past.