Sentence:  If anyone asked us , after we made the remark that the suffering was a bad thing , whether we should think it relevant to what we said to learn that the incident had never occurred and no pain had been suffered at all , we should say that it made all the difference in the world , that what we were asserting to be bad was precisely the suffering we thought had occurred back there , that if this had not occurred , there was nothing left to be bad , and that our <WSD>assertion</WSD> was in that case mistaken . 
Meaning of the word marked with <WSD> tags:  a declaration that is made emphatically (as if no supporting evidence were necessary)