Jan Van Eyck's The Last Judgment.
When I was a graduate student at Miami University working for my master's and Ph.D. in American history with an emphasis on the Progressive era and Great Depression, an essential book was The Modern Researcher, by Jacques Barzun and Henry Graff. Barzun went on to write another important work entitled, From Dawn to Decadence, 1500 to the Present: 500 Years of Western Cultural Life.
By decadence, Barzun, a French-born American historian, didn't mean we had degenerated into orgies and widespread louche behavior (too bad). Rather, he argued that the West had run out of ideas. Look at any skyline and you'll see variations of the same International style architecture that has overrun our cities since the end of World War II. But no revival of Art Deco, the pinnacle of architectural achievement in the 1920s and 1930s. Go to the symphony and you'll hear new compositions that only harken back to the atonal music of the early 20th century.
Even pop culture has stagnated. My unified theory is that everything that's bad is getting worse and everything good is at risk, that keeping people stupid allows for every kind of mischief, especially to our democracy.
More than that, we're entering a new Dark Age. The Dark Ages, as every schoolchild once knew, lasted from the fall of the Western Roman Empire until the Renaissance, Reformation and Age of Enlightenment. Our new Dark Age will be different, yet perhaps share some similarities.