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.

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.

Of course there may well be particular reasons why Spencer rather than others is dead, as there were also particular reasons why he rather than others made such a stir.

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.

Science is intimately integrated with the whole social structure and cultural tradition. They mutually support one other -- only in certain types of society can science flourish, and conversely without a continuous and healthy development and application of science such a society cannot function properly.

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