Sentence:  With more than Quixotic courage and boisterous energy, he not only arrays himself against every belief or institution which wears the aspect of stability or general acceptance, but he continually mistakes the phantasies of his own imagination for actual <WSD>oppugnable</WSD> realities. 
Meaning of the word marked with <WSD> tags:  (rare) Disputable, contestable, refutable, capable of being oppugned.