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.

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.

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.

Sociology should... be thought of as a science of action-of the ultimate common value element in its relations to the other elements of action.

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

If there are four equations and only three variables, and no one of the equations is derivable from the others by algebraic manipulation then there is another variable missing.