'A Kind of Mental Inertia'

From The Waning of the Middle Ages by Johan Huizinga:

"...an assiduous reader of the chronicles and the literature of the fifteenth century will hardly resist the impression that nobility and chivalry occupy a much more considerable place there than our general conception of the epoch would imply. The reason of this disproportion lies in the fact, that long after nobility and feudalism had ceased to be really essential factors in the state and in society, they continued to impress the mind as dominant forms of life. The men of the fifteenth century could not understand that the real moving powers of the political and social evolution might be looked for anywhere else than in the doings of a warlike and courtly nobility. They persisted in regarding the nobility as foremost of social forces and attributed a very exaggerated importance to it, undervaluing altogether the social significance of the lower classes.

So the mistake it may be argued, is theirs, and our conception of the Middle Ages is right. This would be so if, to understand the spirit of an age, it sufficed to know its real and hidden forces and not its illusions, its fancies and its errors. But for the history of civilization every delusion or opinion of an epoch has the value of an important fact...

...This failing to see the social importance of the common people, which is proper to nearly all authors of the fifteenth century, may be regarded as a kind of mental inertia, which is a phenomenon of frequent occurrence and vital importance in history."

'Mental inertia' is the kind of phrase that gets turned around in the mind. One wonders how much one suffers from it oneself. Or how much one's 'epoch' is given over to what, in retrospect, might be clearly seen as delusions of the most untethered sort.