What does the phrase "mankind's essential illness" mean to Simon in "Lord of the Flies"? Give reasons for your thinking.
In chapter 5, Ralph holds an assembly to discuss why they boys are neglecting their duties and urges them to follow through with the agreed upon tasks. Ralph then brings up the topic of the beast and mentions that there is absolutely no malevolent creature on the island. Jack and Piggy both agree with Ralph and testify that something of that nature could not inhabit the small island. However, Simon suggests that maybe there is a beast. Unfortunately, Simon cannot publically express his thoughts, and Golding writes,
"Simon became inarticulate in his effort to express mankind’s essential illness" (126).
Simon is the only boy on the island who understands the true nature of the beast, which is humanity's inherent wickedness. The "essential illness" that Golding references is the sinful, corrupted nature of mankind. Golding holds a pessimistic view of mankind and believes that humans are inherently wicked individuals, which is why the majority of the boys become savages without the rules and regulations of civilization on the island.
In a Biblical context, mankind's essential illness is a result of Adam and Eve disobeying God. Their "original sin" of consuming from the tree of knowledge of good and evil resulted in what is referred to as the Fall of man. Essentially, Golding held similar views of mankind and believed that humans are inherently sinful, wicked beings, which is something that Simon cannot articulate to the group of boys.
Something "essential" is something pertaining to the essence of a thing, and so what Simon imagines is an illness (not necessarily a sickness, the state of being ill, but perhaps a badness, an evil, a nastiness) which is at the very heart of all humans. What Golding's narrator describes Ralph, in the final page of the book, weeping for is "the darkness of man's heart".
The natural evil of humans, then, is something inexpressible: and indeed, the quote above comes from the sentence "Simon became inarticulate in his efforts to express mankind's essential illness". This darkness, this evil, is something difficult to pin down in words: it is the invisible moral weakness in humans that allow the events of the novel, and its descent into savagery, to occur.
So "mankind's essential illness" is not actually Simon's phrase, but Golding's. What Simon is trying to express is that there is a beast (or something, if you like, beastly, on the island) and that, as he says, "maybe it's only us". Simon's meaning is that the beast - the destructive force which scares the boys - is perhaps not an external force, but something dark within the boys themselves: not in a supernatural way, but something fundamentally bad present in human beings.