As "On Liberty" is both a philosophical exploration of liberty and a crash-course in the status of liberty through thousands of years of human society and civilization, Mill makes no shortage of allusions to previous political philosophers, significant literature, and historical civilizations. At the very beginning of Chapter 1, Mill connects Greece, Rome, and his contemporary England together as three examples of empires that "struggled" to balance out "Liberty" with state "Authority":
The struggle between Liberty and Authority is the most conspicuous feature in the portions of history with which we are earliest familiar, particularly in that of Greece, Rome, and England. But in old times this contest was between subjects, or some classes of subjects, and the Government. By liberty, was meant protection against the tyranny of the political rulers. The rulers were conceived (except in some of the popular governments of Greece) as in a necessarily antagonistic position to the people whom they ruled.
Through allusions such as this, Mill contextualizes his philosophical discussion of liberty in the lengthy debate about the ideal balance between state authority and individual freedom that—from his perspective—stretched directly back from the 19th century to the ancient democracies and republics of classical antiquity.
Emphasizing the perceived ideological connection between the Western Europe of modernity and the Greco-Roman civilizations of antiquity was a common feature of Enlightenment and post-Enlightenment philosophy, art, and literature. The allusion to, or even direct invocation of, classical authors was a common way for thinkers like Mill to lend historical weight to their arguments.
Not all of Mill's views have aged well, particularly his attitude toward those societies and people that he viewed as "barbaric." In Chapter 1, when justifying his argument that "despotism" is valuable in order to civilize "barbarians," Mill invokes a historical allusion to the emperors Akbar and Charlemagne:
Despotism is a legitimate mode of government in dealing with barbarians, provided the end be their improvement, and the means justified by actually effecting that end. Liberty, as a principle, has no application to any state of things anterior to the time when mankind have become capable of being improved by free and equal discussion. Until then, there is nothing for them but implicit obedience to an Akbar or a Charlemagne, if they are so fortunate as to find one.
Akbar was an emperor of India in the 16th century, while Charlemagne was the Holy Roman Emperor in the 9th century. Both were responsible for uniting a significant number of disparate states under their empires and therefore qualify as forces of "civilization" by Mill's view. But this allusion does little to temper the severity of Mill's dismissal of "lesser" human societies: he holds the sinister belief that such societies deserve authoritarian rule until they are "capable of being improved."
Although Mill generally argues for the sanctity of individual liberty in his essay, passages like the one above reveal that Mill could not quite escape the ideological influence of British Imperialism and its racist dismissal of indigenous populations that did not conform to a European model of civilization.
In Chapter 2, Mill impresses upon the reader the importance of having some self-awareness about one's worldview and beliefs. Using allusion, he reminds the reader that every age, and every society, has people who believe what they believe to be true according to their faiths and their lived experiences, and it's just as likely that—if one were to be born halfway across the world—they'd find these different perspectives totally acceptable. Ultimately, the most important thing is to remember how every "age" will believe things that, in the future, will seem "absurd," just like past beliefs seem absurd in the present age:
[...] it never troubles him that mere accident has decided which of these numerous worlds is the object of his reliance, and that the same causes which make him a Church-man in London, would have made him a Buddhist or a Confucian in Pekin. Yet it is as evident in itself, as any amount of argument can make it, that ages are no more infallible than individuals; every age having held many opinions which subsequent ages have deemed not only false but absurd; and it is as certain that many opinions now general will be rejected by future ages, as it is that many, once general, are rejected by the present.
Mill's argument relies on allusions to the local religion of various significant civilizations: the Anglican Church in London and the beliefs of Buddhism and Confucianism in Pekin (now Beijing), China. Depending on the time and place of one's birth, one could hold any number of opinions that seem to be absolutely true but that are seen as ridiculous in another place or at a later time.
Mill's observation fits in with his exploration of morality, new ideas, and progress throughout his essay. He is arguing that humanity must be well aware of its fallibility and must never think any of its ideas unassailable, given that the march of time and progress will always bring new ideas and beliefs that conflict with or even entirely supersede the old.
In Chapter 2, Mill argues for the importance of questioning ones beliefs—even those that seem to be facts. In another one of his chain allusions, which Mill uses to contextualize his arguments in the issues and politics of his time, he invokes the Catholic Church and Newtonian Physics to emphasize the importance of this questioning:
The most intolerant of churches, the Roman Catholic Church, even at the canonisation of a saint, admits, and listens patiently to, a “devil’s advocate.” The holiest of men, it appears, cannot be admitted to posthumous honours, until all that the devil could say against him is known and weighed. If even the Newtonian philosophy were not permitted to be questioned, mankind could not feel as complete assurance of its truth as they now do.
Mill believes that the ability to critique and question others for their opinions—and to be open to such critique oneself—is a central component of human liberty. In this double allusion to the Catholic Church, which Mill sees as the least tolerant religious body on Earth, and Isaac Newton's physics, Mill grounds his case in two highly relevant examples: first, if even the Catholics welcome dissent, it must be important. Second, how compelling would Newton's work in physics be, if he refused to listen to any challenges to his work? To Mill, such disagreement is a fundamental driving force for social and political progress.
In Chapter 2, Mill alludes to the story of Jesus Christ's crucifixion. This story, as re-told by Mill, highlights a double irony in the foundation of Christianity: first, Jesus himself was prosecuted as a blasphemer, and second, Saint Paul, eventually one of the apostles, started off as a persecutor of Jesus's disciples:
The high-priest who rent his garments when the words were pronounced, which, according to all the ideas of his country, constituted the blackest guilt, was in all probability quite as sincere in his horror and indignation as the generality of respectable and pious men now are in the religious and moral sentiments they profess; and most of those who now shudder at his conduct, if they had lived in his time, and been born Jews, would have acted precisely as he did. Orthodox Christians who are tempted to think that those who stoned to death the first martyrs must have been worse men than they themselves are, ought to remember that one of those persecutors was Saint Paul.
All of this is to say that beliefs change, and that human judgement is eternally fallible—and therefore that objective truth cannot always withstand assaults from authority. If a figure as influential and benevolent as Jesus can be put to death, Mill argues, then we cannot assume goodness and justice will automatically prevail. Instead, we must take the initiative to think critically and decide for ourselves.
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.
