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 <WSD>idea</WSD> , descending through European democracy , is the source of Marx 's theory of the dictatorship of the proletariat . 
Meaning of the word marked with <WSD> tags:  the content of cognition; the main thing you are thinking about