The history of human civilisation is often presented as a story of progress. We are told about great discoveries, great leaders, great religions, great nations and great revolutions. What receives far less attention is the mountain of corpses lying beneath these achievements.

Throughout recorded history, countless millions of human beings have been killed, imprisoned, persecuted, dispossessed and terrorised by people who were absolutely convinced they possessed the truth. Not a truth. The truth. That distinction matters.

The individual who admits uncertainty can compromise. The individual convinced he possesses absolute certainty often cannot. The moment a belief is elevated beyond criticism, violence becomes easier to justify. The victim ceases to be a person and becomes a problem. The dissenter becomes a threat. The unbeliever becomes an enemy. The heretic becomes a target.

The logic repeats itself so consistently throughout history that it is difficult to avoid the conclusion that certainty itself is among the most dangerous forces ever unleashed upon humanity.

The French philosopher Voltaire understood the mechanism with devastating precision when he warned that those who can make people believe absurdities can make them commit atrocities. The statement survives because history repeatedly confirms it. Human beings rarely commit large-scale atrocities without first accepting extraordinary claims about themselves, their enemies or their mission.

Religion has often provided those claims.

For centuries religious institutions claimed divine authority beyond challenge. Kings ruled by divine right. Priests interpreted the will of heaven. Religious wars erupted across Europe, the Middle East and beyond. Christians killed Christians. Muslims killed Muslims. Christians killed Muslims. Muslims killed Christians. Each side possessed sacred texts. Each side possessed certainty. Each side claimed truth.

The dead inherited none of it.

The philosopher Blaise Pascal observed that men never do evil so completely and cheerfully as when they do it from religious convictions. It is an uncomfortable observation because it strikes at the heart of human self-deception. The greatest crimes are seldom committed by people who believe themselves wicked. The greatest crimes are frequently committed by people convinced they are righteous.

This reality does not apply only to religion.

The twentieth century demonstrated that political ideologies can become every bit as dogmatic as organised faith. Fascism promised national rebirth. Communism promised equality. Revolutionary movements promised justice. Each arrived carrying a vision of a better future. Each produced movements willing to sacrifice human beings for an abstract cause. Entire populations discovered that political salvation could become just as lethal as religious salvation.

One of the most important lessons of modern history is that human beings do not need a deity in order to become fanatics. A flag can be enough. A race can be enough. A class can be enough. A party can be enough. An ideology can be enough.

The social philosopher Eric Hoffer reached a similarly disturbing conclusion when he observed that mass movements can spread without belief in a god but never without belief in a devil. Every movement requires an enemy. The infidel. The heretic. The capitalist. The communist. The foreigner. The traitor. The unbeliever. The enemy changes. The mechanism remains.

Once an enemy has been created, almost anything becomes possible.

The twentieth century offers relentless evidence. Two world wars devastated entire continents. Totalitarian regimes imprisoned millions. Political purges destroyed lives. Ideological conflicts spread across nations. Human beings equipped with modern technology demonstrated a capacity for destruction previously unimaginable. Scientific advances that could have transformed civilisation were repeatedly placed at the service of warfare.

The absurdity of the process has not escaped observers.

The writer H. G. Wells warned, "If we don't end war, war will end us." That warning remains relevant because humanity continues to invest vast resources into developing increasingly sophisticated methods of killing while simultaneously declaring its desire for peace.

No religion has ended war. No ideology has ended war. No empire has ended war. No political system has ended war. Every generation announces that it has discovered the final answer. Every generation eventually discovers that human nature is less cooperative than the theorists claimed.

This does not mean all religions are equally harmful. It does not mean all political systems are equally destructive. It means that every institution claiming immunity from criticism deserves scrutiny. Every leader demanding obedience deserves scrutiny. Every movement claiming ownership of absolute truth deserves scrutiny.

The lesson emerging from centuries of conflict is surprisingly simple.

Ideas are powerful. Ideas shape civilisations. Ideas inspire achievements. Ideas inspire sacrifice. Ideas also kill.

The danger begins when human beings decide that an idea matters more than the people supposedly being served by it.

The moment questioning becomes forbidden, truth begins to suffer.

The moment criticism becomes treason, freedom begins to suffer.

The moment obedience becomes a virtue in itself, humanity begins to suffer.

History's graveyards contain followers of every religion, every ideology, every nationality and every political persuasion. They stand as witnesses to a lesson civilisation repeatedly ignores.

The greatest threat has never been disagreement. The greatest threat has been the conviction that disagreement must be destroyed. That is the price of certainty.

Humanity has been paying it for thousands of years.