Over the last four years, I have visited the United States more than 20 times. I have lectured on philosophy and Islam at numerous academic institutions from Dartmouth to Stanford and at organizations from the Brookings Institution to the United States Institute of Peace. I was invited to a meeting organized by former President Clinton, and I spoke before officials of the CIA.

So when I was offered a professorship at the University of Notre Dame, I did not see it as anything particularly controversial, and I accepted the position as an opportunity for greater engagement and dialogue with Americans.

After the necessary security clearance, my visa was approved in May. We shipped our belongings and were only nine days away from moving when I was informed that my visa had been revoked. Although no explanation was given to us, government officials were quoted anonymously in the media citing the Patriot Act as the legal basis — but without stating exactly what I had been accused of.

The media speculated endlessly; all my detractors’ old and baseless allegations were listed: “Possible terrorist links,’’ “Islamist,’’ and the particularly inexplicable “gentle jihadist.’’ I was accused of being an anti-Semite and of engaging in “double talk’’ by delivering a gentle, moderate message to non-Muslims but a “radical and extremist’’ message to Muslims. My critics pointed to my pedigree — my grandfather was the founder of the Muslim Brotherhood in Egypt.

Time and again I fought to disprove these malicious allegations. But it didn’t work. In 20 years of studying and teaching philosophy, I have learned to appreciate the inherent difficulty in recognizing “the truth.’’ But I also have learned that in the world of mass media, “truth’’ is not based on clarity but on repetition. An assumption repeated three times becomes a fact.

Strange truth indeed! I have written 20 books and 700 articles. Have my detractors read any of them? Are they familiar with my statement on Sept. 12, 2001, calling on Muslims to condemn the terror attacks? Or my condemnations of anti-Semitism?

My record is open and clear. I have no cause for concern. So in September, when the university was advised that I should reapply for my visa, I did so. But Bush administration indicates that no decision is forthcoming in the near future.

This is an affront to justice, to my dignity as a scholar, and it is a violation of my basic human right to know what I am accused of and what proof there is to support it.

So last week I had to make the very difficult decision to resign my post at the university. Now I am waiting for the Bush administration to reveal the results of its investigation, and for my name to be cleared of all the untrue and humiliating accusations I have been subjected to these last few months.

The US government is descending rapidly into a closed and worrisome unilateralism. But America’s ideals are still exemplified by many of its citizens. These ideals were visible in the courageous stance of the University of Notre Dame and all those academic and civic organizations, intellectuals, journalists and ordinary people who defended me. They made it clear that academic freedom should be upheld, even if they didn’t agree with every one of my ideas. They called for openness, transparency and dialogue, and cautioned against censorship rooted in fear and suspicion. It is they who represent the dignity of America.

— Tariq Ramadan’s most recent book is “Western Muslims and the Future of Islam.”