Sentence:  Accordingly we may speak of the Platonism peculiar to Shelley 's poems or the type of Stoicism present in Henley 's `` Invictus '' , and we may find that describing such Platonism or such Stoicism and contrasting each with other expressions of the same attitude or mode of thought is a <WSD>difficult</WSD> and challenging enterprise . 
Meaning of the word marked with <WSD> tags:  not easy; requiring great physical or mental effort to accomplish or comprehend or endure