Three ancient Greek philosophers, after participating in a lengthy argument in a hot day, passed out under the shadow of an apple tree in the Academy. A couple of mine workers passed by when they were napping, so the miners smeared some charcoal on their foreheads.
When the philosophers woke up, they looked at each other and laughed. Every single one of them thought that they didn’t have charcoal smeared on their face, and laughed at the two others laughing at each other. After a second, one philosopher stopped laughing because he deduced that he must have charcoal smeared on his forehead too.
How did he come to that conclusion?