Sentence:  This can only be for one of two reasons : either the two are quite different and will require totally different <WSD>theory</WSD> ( and hence techniques ) , or our existing theories are insufficiently general . 
Meaning of the word marked with <WSD> tags:  a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena