It is that of increasing knowledge of empirical fact, intimately combined with changing interpretations of this body of fact - hence changing general statements about it - and, not least, a changing a structure of the theoretical system. ↗
Special emphasis should be laid on this intimate interrelation of general statements about empirical fact with the logical elements and structure of theoretical systems. ↗
Thus, in general, in the first instance, the direction of interest in empirical fact will be canalised by the logical structure of the theoretical system. ↗