Seeing this now due to the unhappy passing of @TriPolar , whose last post was in this thread…
I don’t dispute the twentieth century as a pick, but surely the 5th century BCE must get some kind of runner-up award. It featured a blossoming of new religions (and thus, probably, new ways of thought) all around the globe:
- 5th century BCE: The first five books of the Jewish Tanakh, the Torah (Hebrew: תורה), are probably compiled.[42]
- 551 BCE: Confucius, founder of Confucianism, was born.[37]
- 600 BCE – 500 BCE: The earliest Confucian writing, Shu Ching, incorporates ideas of harmony and heaven.
- 599 BCE – 527 BCE: The life of Mahavira, 24th and last Tirthankara of Jainism.[43]
- c. 570 BCE: Pythagoras, founder of Pythagoreanism, was born.
- 563 BCE – 400 BCE: Siddhartha Gautama, founder of Buddhism, was born.[44][45][46]
I don’t think we have any good dates for the history of philosophy in the Americas, but I’d be curious what was happening then on this side of the pond.