Lindsey Wasson / Reuters

By April, 2019 was already a bad year for measles. Outbreaks were occurring all over the world, including in Ethiopia, Kazakhstan, Madagascar, Sudan, Thailand, and Ukraine. In the Philippines, more than 200 people, most of them children, had died from measles in January and February. By the end of February, 13 deaths had been reported in Europe. And the European outbreak had already traveled to the United States, where a state of emergency was declared in Washington State. In March and April, emergencies were declared in Rockland County, New York, and New York City, where measles had traveled from Israel. Like the United States, Israel has high overall vaccination rates, but in some communities, many people have chosen not to vaccinate their children.

A public-health emergency allows officials to limit individual liberties. Unvaccinated people were banned from enclosed public places in Rockland County and fined if they failed to comply with mandatory vaccination in New York City. Both measures met immediate legal challenges. One of the legal precedents for public-health emergencies is a 1905 case, Jacobson v. Massachusetts, in which the Supreme Court interpreted vaccination as a form of self-defense, finding that “a community has the right to protect itself against an epidemic of disease which threatens the safety of its members.” Legally, a person carrying an infectious disease is not unlike a person carrying a weapon.

The concept of clear and present danger was once used to defend mandatory vaccination in times of epidemic. And the term conscientious objector, now associated primarily with war, originally referred to people who refused vaccination. Those objectors were not refusing to carry weapons—they were insisting on their right to carry disease. The United Kingdom Vaccination Act  of 1853 required the vaccination of all infants and was resisted by many parents. Later legislation allowed for resisters to be repeatedly fined, and those who could not pay had their belongings seized and auctioned, or were imprisoned.

None of this went over well. In 1898, the English government added a conscience clause to the act, allowing parents to apply for an exemption. This exemption was the precursor to today’s “philosophical” exemptions in the United States, where 15 states allow parents to send unvaccinated children to child care and/or public schools for reasons that are not medical or religious, but philosophical. The conscience clause was vague, requiring only that the objector “satisfy” a magistrate that her objection was a matter of conscience. The new law resulted in thousands of cases of conscientious objection, in some places applying to the majority of all births, as well as a debate about what exactly it meant to possess a conscience.

Before the concept of a conscientious objector was written into law, the term was used by vaccine resisters to distinguish themselves from parents who had simply not bothered to vaccinate their children. The word conscientious was meant to signal that this was an intentional decision made by caring parents. Conscientious objectors argued that a conscience could not and should not be evaluated, and the magistrates themselves were vexed by the problem of whether to demand some sort of evidence to support a claim of conscience. “I don’t understand the act,” one magistrate said in frustration. “I have seen you, and you have told me you have a conscientious objection; I don’t know whether that is enough.” In debating the law, parliamentarians determined that the conscience was very difficult to define.

Today we tend to think of the conscience as an inner voice, or a form of moral intuition. It’s a little cricket whispering in your ear. Your mind talking to itself. The idea that the conscience is an inner source of knowledge has obvious appeal. If your own conscience can tell you what to do in morally significant situations, you don’t need to struggle with others to arrive at justifiable decisions.

Philosophy is not a matter of declaring rigidly held beliefs, but of working out what can be held true in conversation with others. In the Western tradition, going all the way back to Plato, philosophy is based on dialogue. But philosophical exemptions to vaccination laws excuse people from explaining themselves. And the philosophy behind many exemptions remains as remote and ill-defined today as the conscience was in the late 1800s. These exemptions allow parents to offer their fears as justification for not vaccinating their children. The question of whether this is “enough” to justify exposing others to disease is what once frustrated the magistrates charged with judging a claim to conscience. Americans seem to have inherited a folk theory of conscience that confuses firm belief with knowledge. But you can’t know something that isn’t true, though you can certainly believe something that isn’t true. Beyond the confusion of belief with knowledge, our intellectual tradition offers several other theories of conscience. Some explain conscience as a special motivation to avoid wrongdoing or fulfill obligations; others present it as a kind of reflective thinking about the morality of one’s actions.

The philosopher Immanuel Kant argued that people have a duty to themselves to examine their actions, which implies that the moral quality of our actions is not immediately transparent—it must be deciphered by conscience, the “scrutinizer of hearts.”  Kant thought of conscience as an inner judge and used the metaphor of a courtroom to explain its operation. In the courtroom of the conscience, the self is both judge and judged. So, a parent fulfills her duty to conscience, according to Kant, when she judges whether her decision regarding vaccination is guided by fear, self-interest, or genuine concern for duty. Exercising conscience in this sense means taking a hard look at the justifications for your own judgments.

Conscience does not determine whether an action is right or wrong. Instead, conscience calls a person to the witness stand to testify about the process by which she has decided what is right or wrong. Kant’s example is an inquisitor who has condemned a heretic to death. Even if the inquisitor feels certain of his decision, he lacks conscience because he did not examine his own claim to certainty. A conscientious person would not ignore the possibility that he is mistaken when choosing to end another person’s life. To Kant, being conscientious means that you do not make a decision that you cannot prove you are certain about.

Thorough self-examination might not reveal to everyone the true stakes of a decision against vaccination: the risk of exposing infants, cancer patients, and other vulnerable people who cannot be vaccinated to a life-threatening illness. But Kant’s logic still applies: Acting from a belief system that may run contrary “to a human duty which is certain in and of itself” is unconscientious. Conscience demands that the relatively healthy prioritize their duty to protect the vulnerable from disease.

Regardless of what Kant thought, it’s possible to believe that you have diligently examined your own judgments and still fail to consider some essential point. The wide dissemination of misinformation about the risks of vaccination facilitates this failure. And the result is dangerous pockets of unconscientious objection. When an outbreak of infectious disease makes the news and questions are posed about why infected children were not vaccinated, it becomes obvious that the standards for self-examination are communal, just like the criteria for fulfilling our duties to others. As citizens, we have to work out what counts as fulfilling our duties to others with everyone in our community of contagion.

Long before the term immunity was used in the context of disease, it was used in the context of law, to describe an exemption from service or duty to the state. Immunity came into common usage to mean freedom from disease as well as freedom from service in the late 19th century, after states began requiring vaccination. In a peculiar collision of meanings, the exemption from immunity made possible by the conscience clause was a kind of immunity itself. Allowing oneself to remain vulnerable to disease remains a legal privilege today. And the consequences of exercising this privilege are measured in human lives.

The word conscience comes from the Latin verb conscire, meaning “to know with.” Before Kant, the conscience was imagined as a bridge between the self and others. We now need to be conscientious in this sense—engaging with others outside our circle—as well as in Kant’s sense of examining ourselves. Collective knowledge is embedded in the word conscience, along with the possibility that we cannot know alone. Our bodies are not entirely independent, vulnerable as they are to infection from others, and neither are our minds. Conscience is not an isolated space, cut off from the rest of the world. It is what reminds us of our own fallibility, and what makes us accountable to others. As measles spreads across the world, even in places where it is no longer endemic, the meaning of conscience suggests that none of us is exempt from our obligations to others.


Portions of this essay have been adapted from On Immunity by Eula Biss, Graywolf Press, 2014.

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