Sentence:  But they fed what they saw as raw, unguarded <WSD>essentialism</WSD> into work that was anything but naive. Rousseau was a naïf himself; his work as seductively strange to the other modernists as an ancient Iberian sculpture or a child's drawing. 
Meaning of the word marked with <WSD> tags:  The theory that human beings are by nature (i.e. essentially) good and that evil is the product of society.