There are two different senses of rationality of methodological rules: one is instrumental rationality, another is rationality of goals. In the first sense methodological rules are mere means of an apparently neutral true description of a given reality. Such a description, no matter how adequate, involves hidden value-assumptions and may be used for irrational purposes. A different notion of ends-means rationality characterizes methodological rules of critical science which analyses limitations of the given reality from an explicitly stated value-standpoint. The ultimate purpose of such critical research is to produce changes in human behaviour and in objective reality. As in the case of medical activity: diagnosis is followed by therapy. Methodological rules of critical inquiry are only a special case of a general methodology of human practice, the rationality of which presupposes a universal emancipatory goal.