@tsarstepan,
Just quoting Koestler, Tsar. I think he means the collective "we" in the sense of the accumulated knowledge of an entire culture. Individuals may suffer a concussion, come down with amnesia, incur brain damage. But "we" still have the knowledge that was acquired by an earlier generation.
I urge you to read the book that quotation is from; he pretty well demolishes his own argument later on by pointing out that the ancient Greeks of Euclid's time knew far more geometry than the European philosophers of the late Middle Ages.