I'm very keen to examine all the formats that we could have and I've suggested that perhaps we should have one debate with all the parties in, so that everyone can have their say, and perhaps we could have a debate where the two people who could actually be prime minister debate directly with each other.
I don't think you could have a party like Ukip, without an MP, without the Greens, who have got an MP. So there are quite a lot of issues that have to be ironed out.
This differs slightly from the Sunday Times's report last weekend, which suggested that Cameron was open to a "2-3-5" format with a head-to-head debate between himself and Miliband, another with the addition of Clegg, and another with the addition of Nigel Farage and the Greens' Natalie Bennett. Cameron's comments suggest he favours a tête-à-tête with Miliband and a seperate debate with all five party leaders.
His insistence that the Greens would be included is striking. At least one motive is the Conservatives' belief that the fifth party's increased profile would harm Labour and the Lib Dems. Having long complained about the unity of the left compared to the disunity of the right, Tories have told me that allowing the Greens to enter the debate would be another way to depress Labour's vote share in a close election.
It's also worth remembering that were Cameron to debate Miliband, the encounter would need to take place outside of the official election period. As I've noted before, Ofcom rules on impartiality mean it would not be possible to exclude Clegg. Should Ukip be reclassified as a "major party" (as it has been for the European elections), Farage would also have to be included.
But as Labour suggests, Cameron's words may well just be "ducking and diving" designed to prevent the debates from happening at all.