Extremeism is as extremeism says - an incredibly (one might even say extremely) blinkered refusal to consider any other possibility.
If a builder turned extremeist then they might believe that hitting everything with a hammer would get results and continue to believe this despite every pane of glass they tried to fit or every vase they tried to paint shattering to pieces.
It's a wonder the concept manages to survive - but taking heart, it does at least give you hope that as the overwhelming majority of us don't follow such dumb-ass philosophies (at least not all the time ;) ), then we might just have enough brain cells to make it as a species.