Ah yes, the question which seems to fog through the ages. I studied this very question on my own for some time and was left more bewildered and perplexed than before. The truth, as it seems to be, is that people have always been, as you say stupid. All that has changed from then to now is how we are stupid.
The conventional belief is that, in the time of Shakespeare, every man women and child possessed a level of diction chiefly superior to that of todays decorum. But conventional wisdom, as in this case, is usually false. Throughout the ages man has kept an equanimity of inteligence that has always remained the same amount, but as time progresses, shifts from place to place.
I would never debate that the Greek philosophers and scientists of yore were on any account ‘stupid.’ But when you examine their theories in relevance to today’s science, you would see that they had the right idea, but were wrong on many levels. But that branches onto many an argument.
The point is simply- if a simple point does exist- that at one time that smartest of thinkers thought the world was round, so how much inteligence can we account for todays brilliant minds. Think of a maze, the kind we see mice scurrying in on movies, and swishing around in this maze is a fluid with much the same qualities of water. That fluid is man’s inteligence, and throughout the motion of history it has been flowing through the different corridors of the maze; filling some more than others.
We have always kept the same degree of inteligence, it is merely which corridor of the maze we find ourselves in now.