Simon believes the "essential illness" is the dark side of humanity. He knew that in each of us is both the good and evil that men do...it is up to us to decide which side we will follow. Will we give into temptation or will we resist it in the name of goodness and righteousness.
If one looks at Simon's words and actions before he uses the phrase "mankind's essential illness", he has already asked the question "What is it's [the beast] is us?' He is the only character to get away from all the noise and babble of the boys and go into his own shelter to think about the events on the island. All he has seen so far is that civilization has has broken down and no one seems interested in following the rules. Therefore, there must be something innate, within man, that is wrong. Man must have and "essential illness", just as Simon has an illness, which makes all evil come, not from a beast, but from the human heart. After all, Jack is constantly looking for opportunities to take power, Roger is becoming more evil, and the boys treatment of Piggy is becoming more disrespectful,and possibly violent. The young boys flaunt the rules and even Ralph is rude at times. There seems to be sometime wrong in the inherent wiring of mankind that makes him gravitate to evil.
- Throughout the chapters, we have learned that Simon does not like to meddle in the fights, babbles, and hunting between the boys; he would much rather spend time by himself, thinking. After finding inspiration in delivering “mankind’s essential illness”, Simon asks a question: ‘“What’s the dirtiest thing there is?”’ (Golding, pg.96). Although he never had the chance to fully explain his theory, Simon meant that there was no such thing as “beastie”, and the illness originated from the human heart. The essential illness that exists in every human being is evil, the dark side of human-nature. After spending time alone to recall and think about all the incident that have occurred so far on the island: Jack’s growing desire to obtain power, worsening attitude, possibly even violent, towards Piggy, the increasing hostile between the boys; Simon understood where the problem lied. There is both good and evil in a man, and right now on the island, evil is taking over.
It means that humans have a dark side to their nature. And also it means that all evil stems from the human heart not from the beasts.