Sentence:  In the work of every artist , I suppose , there may be found one or more moments which strike the student as absolutely decisive , ultimately emblematic of what it is all about ; not less strikingly so for being mysterious , as though some deeply hidden constatation of <WSD>thoughts</WSD> were enciphered in a single image , a single moment . 
Meaning of the word marked with <WSD> tags:  the content of cognition; the main thing you are thinking about