From all this it follows what the general character of the problem of the development of a body of scientific knowledge is, in so far as it depends on elements internal to science itself.

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.

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.

Talcott Parsons Quotes