"True progress, which has forced back or overthrown barbarous practices and institutions that were the source of infinite suffering for men and women, and has established more civilized relations and styles of life, has always been achieved through a partial, heterodox, distorted application of social theories. Social theories, in the plural, which means that different and even irreconcilable ideological systems have brought about identical or similar forms of progress. The prerequisite was always that these systems should be flexible and could be amended or reformed when they moved from the abstract to the concrete and came up against the daily experience of human beings. The filter at work that separates what is desirable from what is not desirable in these systems is the criterion of practical reason."