According to the late British historian Arnold J. Toynbee the aristocratic class, which was the main source of intellectual creativity in Graeco-Roman society, started loosing that creativity during the 200s BC and started to develop a kind of archaistic, backward-looking groupthink. There were "intellectual" booms during the Roman imperial period, but these were of a mostly religious nature (Neo-Platonic mysticism among the aristocratic elites, Christianity among townspeople).
The lack of creative intellectual activity during the early Middle Ages was because the only literacy was generally limited to clergy, and education was concerned with educating priests and preserving what had survived from the classical past. Only with improving economic conditions starting around 1000 AD that creative intellectual activity started again with the debate on the problem of universals (
http://en.wikipedia.org/wiki/Problem_of_universals ). During the 1200s Western Europeans got access to most of Aristotle's works, triggering the Late Medieval intellectual boom associated with folks like Thomas Aquinas that culminated in the early Italian Renaissance (late 1300s and early 1400s). The fall of Constantinople resulted in a lot of Greeks coming to Italy and brought most of Plato's works with them, causing the boom in Platonist thought that dominated the latter Renaissance (late 1400s to early 1600s) and triggered the Scientific Revolution.
Starting in the early 1600s Western intellectuals, inspired by the Copernican revolt against the authority of Aristotle, now started going beyond their Graeco-Roman predecessors on a huge scale starting with Rene Descartes and culminating in the Enlightenment.