You've spent almost 50 years in politics. Was there a plan?
As a candidate in 1962, I just wanted something to make me take an interest in public life. My plan was to do medical research on the brain.
In the past few years, I have, by looking at the brain's chemistry and the causes of hubris.
You've written about "hubris syndrome", and diagnosed Tony Blair as a sufferer. How do you feel about his appearance during this election?
If I'd been running the campaign, I wouldn't have had him back.
Does Gordon Brown suffer from hubris?
No! He is too much of a tortured soul for that. He is inclined to see the complexity of an argument - at least until he's made up his mind. He's very different from Blair.
Why don't people like him more?
Popularity in politics is a strange thing. It's not always explicable by the mistakes you've made.
Should he resign if there is a hung parliament?
In a coalition, I think Brown would have the decency to step down. It wouldn't surprise me if the new prime minister was Alistair Darling. And the Young Turks - Ed Balls, David Miliband, Ed Miliband - could fight again.
What result are you hoping will come out of this election?
I want the Liberal Democrats to be in a coalition cabinet for a four-year fixed term. That would give the government the credibility and effectiveness to tackle our horrendous deficit. The two old parties deride coalitions because they challenge their power monopoly. But voters shouldn't fear a hung parliament.
If we do have a hung parliament, what lessons should we take from the UK's last one, in 1974?
I was in that government, spending money like water. And when we scraped a narrow majority, the spending carried on. The electorate would have been much better off with a coalition, and the economic situation then was nowhere near as bad as now.
Who will the Lib Dems join forces with?
It looks as if it will be the Conservatives. But Nick Clegg could work with either party.
What about the argument that Labour and the Lib Dems share a common interest?
The Lib Dems championed civil liberties when they were being trampled on by the Labour Party. And they made tactical voting arrangements with the Tories. All credit to them for it.
Some have suggested the Lib Dems are looking to take Labour's place in a two-party system.
That has been the claim, but a hung parliament needs a significant number of Labour MPs. I think they will make it. I hope so.
The campaign you co-founded last year, Charter 2010, makes the case for a coalition government but not for electoral reform. What's your own view?
Proportional representation needs to be chosen by the British people in a referendum. I prefer the single transferable vote - it would be a hell of a fuss to change the system for the alternative vote, which may be even less proportional.
What about the House of Lords?
Well, my position is illegitimate. I mould prefer the Lords to be entirely elected. I'm not a very active member as a consequence.
Thirty years ago, the SDP aimed to "break the mould" of politics. Are we any closer now?
Yes. It's a great tribute to the Liberal Democrat leadership. There is a depth now to the party - 22 years ago, it was a very different picture.
Do you envy the politicians who are at the heart of this election?
What you're really asking is: would I like to be 30 years younger? The answer is yes.
Who are the politicians of the future?
I tend to look left. Within cabinet, Ed Miliband is a rising star. Outside it, Jon Cruddas.
What political decision are you proudest of?
Coming out of semi-retirement to oppose joining the euro. Blair would have taken us in, which would have been profoundly damaging.
And do you have regrets?
When I was a young neurologist, a woman kept begging me to tell her the results of a brain tumour biopsy. I did, and she collapsed, and soon died. I shouldn't have done the PC thing, telling her exactly what she wanted to know.
What about in your political life?
When the SDP lost in 1987, we should have kept going, and then in 1992, with Thatcher gone, we would have held the balance of power and,
I think, ended up with PR, and Labour as the new power. But we didn't. A sad, sad thing.
Are we all doomed?
No. Homo sapiens is a fantastic species. Maybe there is a better one out there. But we're privileged to be part of the human race.