Karl Popper:

Roughly speaking, Marx shared the belief of the progressive industrialist, of the “bourgeois” of his time: the belief in a law of progress. But this naïve historicist optimism, of Hegel and Comte, of Marx and Mill, is no less superstitious than a pessimistic historicism like that of Plato and Spengler. And it is a very bad outfit for a prophet, since it must bridle historical imagination. Indeed, it is necessary to recognize as one of the principles of any unprejudiced view of politics that everything is possible in human affairs; and more particularly that no conceivable development can be excluded on the grounds that it may violate the so-called tendency of human progress, or any other of the alleged laws of “human nature”. “The fact of progress”, writes H.A.L. Fisher, “is written plain and large on the page of history; but progress is not a law of nature. The ground gained by one generation may be lost by the next.”