One of my favorite books is CANDIDE by Voltaire. While staying in St-Germain-des-Pres in Paris, I had to go aound the corner and have a look at the building where Voltaire lived and died. It is on the left bank of the Seine. Just for a little backstory, Voltaire, the great French philosopher, amassed a library of more than 21,000 books with his favorite subjects being metaphysics, moral philosophy, physics, history, critical deism... He wrote his satire, CANDIDE, in 1759, during the Age of Enlightenment. It was considered his magnum opus.
Voltaire rejected a common theory of the time, that "all is for the best in the best of all possible worlds." He argued that this theory of optimism was wrong - that if this were the best possible world, then it would surely be better than it is, citing natural disasters, war, catastrophes etc. Thus, Candide was born of Voltaire's writings and discussions on this subject.
Candide did set out and travel the world, full of hope and optimism, looking for the good in man. An epic journey, the end finds Candide returning home, "to cultivate his garden." I won't give more away than that.
And, about another modern credo, the glass is either half empty or half full, I've always said, it's half. Guess that makes me a realist.