At the beginning of Chapter 1, Mill lays out a brief overview of why people have traditionally looked to an authority figures for protection. In this case, he uses a series of violent similes and metaphors to demonstrate the complicated role that such figures can play:
Their power was regarded as [...] a weapon which they would attempt to use against their subjects, no less than against external enemies. To prevent the weaker members of the community from being preyed upon by innumerable vultures, it was needful that there should be an animal of prey stronger than the rest, commissioned to keep them down. But as the king of the vultures would be no less bent upon preying on the flock than any of the minor harpies, it was indispensable to be in a perpetual attitude of defence against his beak and claws.
Although Mill has already admitted before this that authoritarian power can be necessary, these literary devices make clear that their power can also be deadly. Mill uses his simile to compare a ruler with a weapon, and, through metaphor, to an animal of prey that could protect "weaker" citizens from "innumerable vultures" (any number of stronger citizens or threats they might face).
Though this might seem to be an absolute good, any such animal of prey could turn also against the weaker populace they have been set up to protect. Thus, Mill emphasizes the importance of a careful, guarded, naturally suspicious relationship between a population and its ruler. This is a vital example of Mill's give-and-take understanding of liberty and authority in his essay.
Occasionally in "On Liberty," Mill leans heavily into figurative language in order to make his point, although most of his work is relatively plain and unadorned. This passage, from Chapter 2, is one notable exception, in which Mill uses similes and metaphors of light, fire, and nature to emphasize how powerful truths eventually escape even the harshest persecution:
Socrates was put to death, but the Socratic philosophy rose like the sun in heaven, and spread its illumination over the whole intellectual firmament. Christians were cast to the lions, but the Christian church grew up a stately and spreading tree, overtopping the older and less vigorous growths, and stifling them by its shade. Our merely social intolerance kills no one, roots out no opinions, but induces men to disguise them, or to abstain from any active effort for their diffusion.
Though Socrates died, his philosophy lived on with such power, Mill contends, that has shone through ages like the glare of the sun. Christians, for their part, may have been persecuted—and Jesus himself may have been executed—but the metaphorical tree that is Christianity could not be felled. Instead, by Mill's metaphor, this tree grew until it blocked out other religions like a large tree blocks out the sun from other smaller trees in its shade.
Later in this same section, Mill continues to use the figurative language of fire:
With us, heretical opinions do not perceptibly gain, or even lose, ground in each decade or generation; they never blaze out far and wide, but continue to smoulder in the narrow circles of thinking and studious persons among whom they originate, without ever lighting up the general affairs of mankind with either a true or a deceptive light.
Though these "heretical" ideas may not conjure the celestial inferno that Socrates's philosophy has, they nonetheless prove to be durable. Like smoldering coals in a fireplace, they persist, although they do not break into the mainstream.
These passages are vital examples of Mill's exploration of how humanity's ideas take root and spread in the face of attempts at censorship by church, state, or social tyranny. Progress—even the progression of heresy—will always burn on, but it can nonetheless be hindered if authority tries to "smother" it out. New, challenging ideas, Mill argues, must always be entertained.
In a few instances in "On Liberty," Mill uses the metaphorical language of sleep to describe the complicity that people tend to fall into when they believe that an opinion has become settled fact.
In Chapter 2, writing about religious dissenters, who have maintained the "vitality" of their beliefs because of constant questioning from more mainstream faiths, Mill notes that in the case of opinions that go largely unchallenged, "both teachers and learners go to sleep at their post, as soon as there is no enemy in the field." He uses the language of sentries at war who nod off without risk to describe unchallenged beliefs, suggesting that people will become complacent unless directly challenged.
Later in the same chapter, Mill makes an allusion to an aphorism by Arthur Helps, a clergyman and contemporary of Mill's:
The fatal tendency of mankind to leave off thinking about a thing when it is no longer doubtful, is the cause of half their errors. A contemporary author has well spoken of “the deep slumber of a decided opinion."
Throughout "On Liberty," Mill is deeply concerned with the fate of any society that finds itself in such a "slumber"—it could be said that he writes his essay in a bid to help humanity stay awake. To that end, passages like these are key moments in which Mill explores the conditions that lead to new ideas and social progress, in the hopes that humanity will keep thinking big, challenging thoughts.
Part of Mill's mission in "On Liberty" is to understand the conditions that lead to originality, or genius—in other words, how it is that humanity can create new ideas. In this passage, from Chapter 3, he uses a metaphor of mechanical production (or industrialized labor) to emphasize how "persons of genius" are able to escape from conformity:
Genius can only breathe freely in an atmosphere of freedom. Persons of genius are, ex vi termini, more individual than any other people—less capable, consequently, of fitting themselves, without hurtful compression, into any of the small number of moulds which society provides in order to save its members the trouble of forming their own character. If from timidity they consent to be forced into one of these moulds, and to let all that part of themselves which cannot expand under the pressure remain unexpanded, society will be little the better for their genius.
This passage connects to Mill's thematic exploration of conformity throughout his essay. Society provides metaphorical "moulds" from which most people tend to be cast, so that they don't need to bother having the trials and tribulations faced by someone who is actually original. Nonetheless, it is vital that some should strive to defy this industrious drive to conformity in order for new, radical ideas enter people's minds.
In Chapter 4, Mill addresses laws that constrain citizens' private behavior. At one point, he discusses the practice of prohibition, or banning the consumption of alcohol. Using a metaphor, Mill identifies this as a dangerous legal precedent because the logic of such a prohibition could be used to constrain any behavior deemed "offensive" by someone else (or society at large).
So monstrous a principle is far more dangerous than any single interference with liberty; there is no violation of liberty which it would not justify; it acknowledges no right to any freedom whatever, except perhaps to that of holding opinions in secret, without ever disclosing them: for, the moment an opinion which I consider noxious passes any one’s lips, it invades all the “social rights” attributed to me by the Alliance. The doctrine ascribes to all mankind a vested interest in each other’s moral, intellectual, and even physical perfection, to be defined by each claimant according to his own standard.
In describing a "noxious" opinion, Mill calls back to his discussion of alcohol and conflates the literally noxious powers of alcohol with the notion of noxious, or dangerous, ideas or ideology. One of the risks of social tyranny is that any sort of dissenting opinion could get treated like ideological "alcohol" and face similarly prohibitive legislation.
At the closing of "On Liberty," in Chapter 5, Mill warns the reader that society will not be able to progress, and humanity will not be able to flourish, if a state authority looms large over it. In the language of machinery and industry that echoes the post-Industrial Revolution time of Mill's writing, he compares a well-functioning authoritarian state to a machine that runs smoothly but can create nothing:
...a State which dwarfs its men, in order that they may be more docile instruments in its hands even for beneficial purposes—will find that with small men no great thing can really be accomplished; and that the perfection of machinery to which it has sacrificed everything will in the end avail it nothing, for want of the vital power which, in order that the machine might work more smoothly, it has preferred to banish.
An authoritarian state may find that it can run more smoothly—like a well-oiled machine, in Mill's metaphor—if it deprives its citizens of their natural liberties, but it won't amount to anything great in the process. By sacrificing liberty in the name of authority, Mill argues, we also smooth over the differences and disagreements that are the "vital power" of societies that actually progress. These imperfect societies are where "great things" can happen, thanks to the flourishing of individual genius or originality that can only occur when individual freedoms are protected.
