Two weeks ago, during the Liberal Democrats' conference, David Cameron wrote a sweet-scented address to Lib Dem voters on these pages. He suggested that we should all unite – under a Conservative banner – to reject the Labour party. It was the political equivalent of a big group hug – let's all be friends, and all will be well.

So today, on the eve of the Conservative conference, here's my reply. I agree that creating rigid dividing lines between parties where they don't exist is silly. Beyond all the ya-boo of Prime Minister's Questions, consensus between parties can still occur. I was pleased when Alistair Darling finally agreed with Vince Cable that Northern Rock should be nationalised, despite the objections from George Osborne. I was pleased when David Cameron belatedly joined the campaign to give justice to the Gurkhas. So parties do manage to agree with each other from time to time.

But does that mean that political differences don't matter any more? Of course not. As people search for alternatives to Labour, those differences are more, not less, important. The key question facing the British people is not whether they want change from Labour, but what kind of change? And Cameron and I propose radically different visions of the Britain we want to live in. The Conservatives have learnt to parrot the language of change, but they won't come clean on their real intentions. They talk about a broken society, but want to spend billions on a tax cut for millionaires while doing nothing for people on ordinary incomes. Forty per cent of children in poverty live in one-parent households, yet the Conservatives want to provide tax breaks for married couples only. They talk the green talk, but stand shoulder-to-shoulder with climate-change deniers in Europe. For all the rhetoric on cleaning up politics, they won't even come clean on whether their most important donor pays full taxes in this country.

Cameron's strategy is simple and, from his point of view, entirely rational: he hopes to breeze into Number Ten on the back of popular despair about Gordon Brown. But I don't think politicians should inherit power. I believe we should earn it. Rocketing unemployment. The fiscal deficit. Climate change. International terrorism. Our rotten politics. The scandal that one in four 11-year-olds cannot properly read and write. None of that will be solved by airbrushing out political differences. The only way we are going to solve these challenges is if we have a serious debate now about the choices we face.

The Liberal Democrats are doing just that. When Gordon Brown let house prices rocket and personal borrowing get out of control, when he let the bankers gorge themselves on bonuses, David Cameron's Conservatives said yes and only the Liberal Democrats said no. We are the only ones with radical plans to transform our tax system so that people at the very top make a fair contribution reducing the burden for everyone else. Alone in British politics we want to take big money out of party funding altogether, abolish the arrogance of MPs in safe seats by introducing fair votes. And I will never stop arguing for Britain to stand tall in Europe, rather than whingeing from the sidelines.

Differences matter. To pretend they don't is a cynical sleight of hand to deny people the choice they deserve at the next election. A choice between a Conservative party that will say whatever it thinks people want to hear, and the Liberal Democrats who stand for a Britain that is fairer, greener and stronger in the world. A choice between fake change and change for good.