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 <WSD>is</WSD> a difficult and challenging enterprise . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)