Sentence:  So impressive were those serious years of study at the university that Hans later wrote , `` to be perfectly free , the young man must revel in the great kingdom of <WSD>thought</WSD> and imagination ; there is a struggle there , in which , if he falls , it is easy for him to rise again , there is freedom of utterance there , which draws after it no irreparable consequences on society . 
Meaning of the word marked with <WSD> tags:  the process of using your mind to consider something carefully