The events of 4 June 1989, when the Chinese government deployed the full might of its military to purge Tiananmen Square of students who’d been peacefully protesting there, have become known in China as the “June Fourth Incident”. Thirty years on, it is still thought of as an “incident”, a one-off event. In fact, it was part of a political movement in which every major Chinese city participated. To this day, a complete definition of 4 June 1989 as a historical event has not been realised, because defining a historical event requires not only the full facts but also multiple perspectives. And in its aftermath, the Chinese government intensified its oversight of free expression in China, deploying various tactics to suppress, arrest, detain and imprison anyone who spoke about “June Fourth”. It remains the most taboo and politically sensitive topic in China, much like the questions of Tibet and Xinjiang for the Chinese Communist party (CCP) and its machine of propaganda and censorship. Yet the facts and significance of “June Fourth” are not discussed in China. The exact events, the persons responsible for issuing directives, the methods of execution, the number of people killed and detained, and the killers responsible for the cumulative political decisions remain unclear.

The memory of the past is an individual’s property. To deny it is to obliterate humanity.

What now, 30 years on, is its significance? The need to examine this question is vital, rational and urgent. If the CCP relied on violent revolution to overthrow the previous regime and establish its legitimacy, then “June Fourth” once again overthrew the legitimacy of the ruling party. The Communist party is a regime that used violence to supplant dialogue, directing its army and tanks against unarmed citizens to maintain its existence. Despite attempts to cover up, whitewash and misinterpret “June Fourth” over the past 30 years, from the moment the first bullet was fired that day the regime’s legitimacy was compromised. Nothing can change that.

On 4 June, CNN’s 24-hour live broadcast conveyed the event and its developments to any audience that could receive its signal. I watched from New York. Viewers in New York probably witnessed a more comprehensive version of the incident than my family in Beijing. In New York, I organised and participated in many demonstrations of solidarity with the students in Tiananmen Square, protested before the Chinese consulate, and took part in a hunger strike at the United Nations.Why does a political power attempt to suppress reality? I have always wondered about totalitarian regimes’ fear of facts. As a political dissident, I insist on seeking the truth and resist attempts to change my memory of events. Because facts constitute the foundation of my understanding of the world. Upholding reality is a precondition for the mind to function. Otherwise, the world before us is disordered and chaotic; a world gone mad.

Why do autocratic and totalitarian regimes, in fact most forms of power, fear facts? The only reason is because they have built their power on unjust foundations. Once facts are established, justice will be restored. And this is the greatest fear of powerful regimes. This is true not only of China, North Korea, or most non-democratic societies, but also some societies with democratic frameworks. When I consider the experience of whistleblowers such as Edward Snowden, Chelsea Manning or Julian Assange, they remind me of my time living in a totalitarian society that suppresses and whitewashes fact, creates no-go zones and fears the light of public disclosure. Even if the lives of an entire generation are wiped out, no prisons and no amount of lies or censorship can expunge or conceal the facts. This is why memory – individual and collective – is such an important part of civilisation. To remove the memory of the past is to rob what is left of an individual, because our past is all we have. Without it, there is no such thing as a civilised society or nation. Any attempt to destroy, remove or distort memory is the act of an illegitimate power.

China is a society without citizens. It is dominated by the CCP. And even after 70 years in power the government still does not trust its people: 1.4 billion have never in those 70 years had the opportunity to vote for their rulers. As a result, there is no freedom of speech and information. The memory of the past is an individual’s property. Its details are the veins carrying blood in the body, giving life to truth. To deny them is to obliterate humanity. Happiness, sorrow, wealth or poverty is all we possess. Once that is taken away, we simply have no future: when there is no past, the word “future” loses its meaning.

When we talk of the past and of fact, it is essential to emphasise the importance of freedom of speech. When facts are changed, freedom of speech does not exist and has no meaning because this freedom cannot exist without an individual’s understanding, vision, emotion and interpretation. What we call social justice could never exist without open discussion in the public sphere because fairness and justice are necessary for public welfare and to maintain a harmonious society. Whenever social justice is missing, there will be crisis and tragedy. This is why we cleave strongly to fact and refuse to forget. This is how we give definition to an individual’s mind, and why we must protect the dignity of being.

What occurred on 4 June is not merely a Chinese issue. It is not simply an event that happened 30 years ago. Injustice is timeless. It haunts us and affects our state of mind until the day justice is served.

At the same time, the tolerance of injustice and distorted information is an act of encouragement and complicity. Such tolerance allows authoritarian regimes to transgress any red lines. This is exactly what happened after “June Fourth”, when the west bought into the excuse that Chinese society would become more democratic after it became richer. China has become wealthier and more powerful on the world stage, but it has never matured into pluralism or democracy. It continues to reject fundamental values of openness, social justice, fair competition and freedom. We will all pay the price for this failure.

• Ai Weiwei is a contemporary artist, activist and advocate of political reform in China