From Surprised by Joy - by C.S. Lewis - with reference to his friendship with Owen Barfield (my punctuation and emphasis):
"[Barfield's] counter attacks destroyed forever [an element] in my own thought.
"In the first place he made short work of what I have called my 'chronological snobbery', the uncritical acceptance of the intellectual climate common to our own age and the assumption that whatever has gone out of date is on that account discredited.
"You must find out why it went out of date.
"Was it ever refuted (and if so by whom and how conclusively) or did it merely die away as fashions do?
"If the latter, this tells us nothing about its truth or falsehood. From seeing this, one passes to the realization that their own age is also 'a period', and certainly has, like all periods, its own characteristic illusions.
"They are likely to lie in those widespread assumptions which are so ingrained in the age that no one dares to attack or feels necessary to defend them."
Why did something go out of date: was it ever refuted? Or to people disbelieve it merely because it is old-fashioned.
Some of the greatest intellectual shocks of the life have been recognizing that old ideas were never refuted but merely died away, to be replaced by something with novelty value but little or nothing else to recommend them - merely because of fashion, because that is the nature and definition of fashion.
I came across numerous examples in science (IQ research) medical history - especially psychiatry (the electroencephalogram); and the biggest example in philosophy - where the vast and coherent synthesis of Aquinas was progressively dismantled and discarded for ever smaller, more partial and less coherent systems.
Of course the process is most obvious in the arts over the past century, where the decline in quality is impossible to hide.
And then of course there are morals...