Logic, made famous by philosophers such as Aristotle or Descartes, is the systematic study of reasoning.
Logic was discovered by someone living in a cave a really long time ago because he was sick of everyone doing stupid things like dying and other mundane daily occurences. Much later, a few thousand years actually, some ancient Greek guys decided logic was pretty awesome and wrote a bunch of books about it. Unfortunately, no one except those with already-superior logic skills understand their roundabout, ass-backwards writing style that clearly was not meant for average mortals.
Then logic disappeared for a while and there were some corrupt popes, some feudalism, some feudal lords, some corrupt popes who were feudal lords, some corrupt popes' sons who were feudal lords and became popes... you get the idea. Apparently, they didn't get the idea and they had silly things like crusades to find some cup and take some land and self-flagellation to repent for whatever sins might have infected them with that plague that nearly destroyed Europe. Of course this was a more innocent time when hacking up Muslims in the Holy Land was a pretty cool idea and being bled with leeches was the only way to cure a cold.
Some years later, some crazy people decided it would probably suck if the Turks got hold of all that random junk Constantine had dumped in Constantinople a thousand years before and the Renaissance began. Suddenly, there was logic again. All sorts of shenanigans ensued, such as: some guy thinking people might be able to fly, Turks not taking over the Western world, Spain thinking they were awesome, America telling the Brits to get back on their boats, and French people chopping off heads by the dozen.
Logic is not to be found in modern society. Due to our lack of logic as a species, we should expect more crusades soon and probably bleeding with leeches to drain evil fluids. Additionally, we may see feudalism become the common socioeconomic system and self-flagellation might be the new cool thing to do in a few years.
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones." - Albert Einstein
He knows what's up.