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.

The system becomes logically closed when each of the logical implications which can be derived from any one proposition within the system finds its statement in another proposition in the same 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.

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.

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