Theory not only formulates what we know but also tells us what we want to know, that is, the questions to which an answer is needed.

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.

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.

But the scientific importance of a change in knowledge of fact consists precisely in j its having consequences for a system of theory.

It is probably safe to say that all the changes of factual knowledge which have led to the relativity theory, resulting in a very great theoretical development, are completely trivial from any point of view except their relevance to the structure of a theoretical system.

Spencer's god was Evolution, sometimes also called Progress.