What you can't say
Paul Graham has a seminal essay (paulgraham.com/say.html). A short quote by C. S. Lewis touches on the same area:
Every age has its own outlook. It is specially good at seeing certain truths and specially liable to make certain mistakes. We all, therefore, need the books that will correct the characteristic mistakes of our own period. And that means the old books. All contemporary writers share to some extent the contemporary outlook—even those, like myself, who seem most opposed to it. Nothing strikes me more when I read controversies of past ages than the fact that both sides were usually assuming without question a good deal which we should now absolutely deny. They thought that they were as completely opposed as two sides could be, but in fact they were all the time secretly united—united with each other and against earlier and later ages—by a great mass of common assumptions. We may be sure that the characteristic blindness of the twentieth century—the blindness about which posterity will ask, “But how could they have thought that?”—lies where we have never suspected it, and concerns something about which there is untroubled agreement between Hitler and President Roosevelt or between Mr. H. G. Wells and Karl Barth. None of us can fully escape this blindness, but we shall certainly increase it, and weaken our guard against it, if we read only modern books. Where they are true they will give us truths which we half knew already. Where they are false they will aggravate the error with which we are already dangerously ill. The only palliative is to keep the clean sea breeze of the centuries blowing through our minds, and this can be done only by reading old books.
(Disclaimer: I don't know much about C.S. Lewis beyond the above quote)
It's funny because we can obviously find at least two other categories to read as palliatives. C. S. Lewis was open to reading different times, but I presume he still only exposed himself to the perspectives of European men, since that… is what tends to happen when an European reads old books. Different times, but not different places, nor different perspectives in his own time, like those of women or minorities or other cultures. The scary part is I can believe that those palliatives didn't even occur to him.