Karl Popper on the importance of argument

From Letters to a Young Contrarian by Christopher Hitchens:

I pass to an observation of the late Sir Karl Popper, who could himself be a tyrant in argument but who nonetheless recognised that argument was valuable, indeed essential, for its own sake. It is very seldom, as he noticed, that in debate any one of two evenly matched antagonists will succeed in actually convincing or "converting" the other. 

But it is equally seldom that in a properly conducted argument either antagonist will end upholding exactly the same position as that with which he began. Concessions, refinements and adjustments will occur, and each initial position will have undergone modification even if it remains ostensibly the 'same'.