Gibbon’s Decline and Fall begins with the death of Marcus Aurelius in 180 AD. It was all downhill from there.
Besides being a well-regarded emperor who was succeeded by an ill-regarded son, Marcus was a philosopher. Reading the works of Epictetus turned him into a devoted stoic as a young man. Marcus’ book Meditations remains the sterling example of the stoic mindset: civility, moderation in all things, and above all, taking triumph and tragedy with the same quiet dignity.
Marcus also had a bit of the economist in him. Despite predating Adam Smith by sixteen centuries, Meditations contains an excellent example of opportunity costs. Only the law of demand is more important in the economist’s toolkit. As a way of saying “mind your own business,” he writes:
Do not waste what remains of your life in speculating about your neighbours, unless with a view to some mutual benefit. To wonder what so-and-so is doing and why… means a loss of opportunity for some other task.*
*Meditations, III.4; trans. Maxwell Staniforth.