The televised leaders' debates have propelled Nick Clegg and his Liberal Democratic Party up the popularity polls. They are now a close second to the Conservatives and may well be part of the next government. Yet most voters know little about the Liberal Democrats. What do they stand for? What kind of country do they want Britain to be?

The answer is written in big colorful letters on the cover of their manifesto: "Fair Taxes, a Fair Chance, a Fair Future, a Fair Deal." The LibDems, like Labour, seek "a future fair for all."

My six-year-old daughter will be delighted. Whenever she cannot stay up late or have another ice cream or otherwise get her way, she declares the situation "unfair." If the LibDems form a government, she will never again be disappointed.

No, that's not right. She is too young to understand what "fair" means. It does not mean having your way. It means … Well, what does it mean? When Mr. Clegg or Gordon Brown promise to make Britain fair, what should we expect from them?

The answer is uncertain because the concept of "fair" is ambiguous. It is sometimes used to mean treating people equally. Call this the Aristotelian conception (for reasons you can guess). Yet it is sometimes used to mean treating people unequally so as to bring about an equal outcome. Call this the Marxist conception.