Most human beings didn’t know what their faces looked like. The familiarity with our own looks that we take for granted because of mirrors and photography was unknown for centuries. Imagine for a moment what it would be like to have your face unknown to you, and all knowledge of it derived from what others said. We occasionally see our faces reflected in still water, but we bring the knowledge we have from mirror and photo to that experience. Rarer still, and more recent historically, people saw themselves in polished metal. There are many living today who have still never seen their own faces with any clarity.
One school of thought (Alfred Crosby, Alan MacFarlane) dates the beginning of the Renaissance to the last quarter of the 13th C, with the improvements in glass and mirrors in northern Italy (Venetian mirrors were the first widespread mirrors of excellence). As one of the key features of the Renaissance was supposed to be the emergence of the individual, this makes a sort of intuitive sense.
I don’t mean to be reductionist. It is a favorite sport of historians to try and identify the one unnoticed factor that changed a culture, or even the world. Double-entry bookkeeping, Toledo steel, and a dozen other advances have been proposed as the key. I don’t believe in the Renaissance anyway, so I won’t offer an opinion on its putative beginning. (For the record, I believe in a period called the Enlightenment even less than one called the Renaissance.) Major technological changes were always coming along and remaking Europe from the 8th to the 20th Centuries: the moldboard plow, the stirrup, vaccination.
Yet I ask you to consider this one, this improvement in glass and silver backing which allowed people to see who they were – to see of themselves what others had always seen. More than the day before, you as an individual could see you existing without depending on the village to tell you who you were.