Sentence:  And it is clearly argued by Lord Percy of Newcastle , in his remarkable long essay , The Heresy of Democracy , and in a more general way by Voegelin , in his New Science of Politics , that this same Rousseauan idea , descending through European democracy , is the source of Marx 's <WSD>theory</WSD> of the dictatorship of the proletariat . 
Meaning of the word marked with <WSD> tags:  a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena