The answer to this question changes through time and across different political terrains.
The battles of liberal democracy against the Nazis and the communists were battles about ends, rather than means. We wanted freedom; they wanted something quite different. And the same is true of the current struggle between liberal democracy and theocratic islamism. We want freedom; the theocratic islamists want something quite different.
But when it comes to the battle of ideas between the centre-left and the centre-right in mainstream British politics, what we are arguing about is much more a question of means than a question of ends.
Nothing illustrates this better than the seminal work that Iain Duncan Smith's policy group has done as part of the Conservative Policy Review. If you read Ian's hugely impressive twin volumes -- Breakdown Britain and Breakthrough Britain -- what you find is a clear identification of progressive goals which anyone from the centre-left would agree with, allied to a series of policy prescriptions for achieving those goals which are unmistakably from the centre-right.
Iain's overriding aim is emancipation from poverty. What could be more progressive, more shared as a goal with the centre-left, than that? But his policy prescriptions do not involve the vast centralised bureaucratic schemes beloved of the Brownites. They rely instead on strengthening the family and making society more responsible. His aim is to ensure that the individuals in poverty are supported by others around them, and are thereby given the power to lift themselves out of poverty. In short, he adopts a characteristically Conservative approach to the means of achieving the progressive goal of eliminating poverty -- social responsibility in place of state control.