There is a credibility decay on positive knowledge among the social scientists and particularly among the psychologists. Certainly different prejudices dwell upon this phenomenon. First and principal the usually made identity of positive and natural knowledge. This belief is also fed by many others such as that one which assumes that every quantification, which is a consequence of positive knowledge,follows the model of the Physical Sciences. We consider that all these prejudices do not keep in mind the work done in (...) the last decades on the Theories of Measure. We consider also that psychophysical methods which have been introduced by Stevens present a stimulating program for the behavioral sciences and they open a new frontiere to the positive knowledge. (shrink)