Special emphasis should be laid on this intimate interrelation of general statements about empirical fact with the logical elements and structure of theoretical systems.

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.

That is, a system starts with a group of interrelated propositions which involve reference to empirical observations within the logical framework of the propositions in question.

Now obviously the propositions of the system have reference to matters of empirical fact; if they did not, they could have no claim to be called scientific.

In so far as such a theory is empirically correct it will also tell us what empirical facts it should be possible to observe in a given set of circumstances.

Empirical interest will be in the facts so far as they are relevant to the solution of these problems.