It was one of the most dramatic moments of my 22 years in the House. We were about to go to war, amid huge controversy. One of the most senior members of the Government had just taken the principled and rare decision to resign. And his speech was Robin at his very best: forthright, intelligent, eloquent, prescient - quite how prescient we now know, with hindsight - and totally in command of the House. As he sat down, the chamber erupted into applause, something not normally seen or heard.
Robin was one of the few speakers who could fill the Commons simply by the appearance of his name on the annunciator's screens. He was one of the great Parliamentary orators. He once said to me that in the Commons he felt as if he was where he belonged. He felt at home there.
He relished the forensic theatricality of adversarial Commons debate: something that made him a formidable figure in Opposition over the Scott Inquiry, after five hours studying the report in a sealed room, he was able to deliver a faultless analysis of Government failure and error which helped to pave the way for Labour's subsequent electoral victory.
Despite his rebellion over Iraq - because he felt in his bones it was the wrong thing to do - Robin was a Labour man through and through. In the recent general election campaign, he criss-crossed the country campaigning on behalf of Labour candidates, whatever their views or their records, making the simple case: the Iraq war was a mistake; he, and many others in the party had opposed it strenuously; but despite that mistake it was infinitely better to have a Labour Government than not.
It is hard to imagine that Robin - had he lived - would have been lost forever to high office. He had such talent and wisdom - and ultimately such devotion to the Labour cause and to his belief in social justice - that it would have been a shameful waste to leave him out of Government forever.
He had, after all, been an outstanding Government minister. As foreign secretary he was a genuine internationalist: passionately pro-European, but seeing Britain's place in the wider world too. His declared intention to include an "ethical dimension" in Britain's approach to foreign policy was much mocked by his opponents in the press, but it represented a brave attempt to cast our country's relations with the rest of the world in a moral light as well as in terms of pragmatic realpolitik. It deserved a better hearing, and will in time come to be regarded as something of a historic innovation.
When he became Leader of the House of Commons immediately after the 2001 election, it wasn't a post he had sought - or at first wanted - but, in typical Robin fashion, he set himself the task of making something of it. He succeeded spectacularly well. He became a great reforming Leader of the House, making major changes, but doing so in a wholly practical way. Some of those reforms - especially over sitting hours - were ultimately to prove too controversial, but most have endured.
In the last few years, Robin's legacy has become even broader. In his speeches and articles he was setting out a vision of libertarian, democratic socialism that was beginning to break the sometimes sterile boundaries of "old" and "New" Labour labels. Had he lived, he would not only have continued to be a combative champion for his Party, but would have been one of its reforming philosophers too.
With Robin's untimely death, Parliament and public life have lost one of the bravest, wisest and most compelling voices. The Labour cause has lost one of its most powerful advocates. And those of us who knew Robin as a close colleague have lost a good and valued friend.
As everyone knows, he loved horses and horse racing - and was one of the best tipsters in the business. He was also immensely proud of his Scottish roots, and he loved Scotland's history and its mountain landscape - a passion that I shared with him. A few years ago, Robin gave me a book of essays and stories about the Scottish hills; it's a volume that I will now treasure with more pride and more sorrow than I ever thought possible.
It is hard to think of the political landscape without Robin Cook as an important part of it. He gave voice to his values and beliefs more tellingly than anyone else could hope to do. We are all diminished by his loss.