If observed facts of undoubted accuracy will not fit any of the alternatives it leaves open, the system itself is in need of reconstruction.

A theoretical system does not merely state facts which have been observed and that logically deducible relations to other facts which have also been observed.

The importance of certain problems concerning the facts will be inherent in the structure of the system.

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

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.

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.