Our face-analysis software is so acute that (as someone pointed out in a recent Warhol documentary) some internationally-known faces can be recognised from a caricature drawn with only a few lines. Hitler, Abe Lincoln, Chaplin, and possibly Warhol himself. We need very few clues in order to recognise a face, once known. This is why we can recognise a friend a hundred yards away when it would be impossible to clearly see all the details of his or her face.
This is also why people believe they can 'see' the face of someone whose features have been pixelated (usually to conceal their identity in TV interviews) by squinting.
Squinting reduces the visual clues available and simulates the 'blur' we experience when looking at someone far away or in dim conditions. At this point our software takes over and fills in the features that the brain is starved of.
This means that our minds are filled with thousands and thousands of faces, comprised of family, friends, fellow-students, work-colleagues, neighbours, people who work in shops and restaurants we visit, doctors, dentists and a disproportionately large number of TV and film personalities, some of whom are not even human.
There are some people whose faces we carry whom we know nothing about. They may catch the same bus or live in the same area. We may not have spoken to them, or even exchanged a smile of recognition, and yet, we carry their face around in our heads, seemingly indefinitely.
This also means that they carry our faces around with them too.
Now there's a thought.