Meanings of unsurmountable difficulties in English

I suppose there are unsurmountable difficulties in making people understand.

And the apparently unsurmountable difficulties, through which these were essayed, makes their even partial development more remarkable still.

I fear unsurmountable difficulties in protecting and transporting our provisions, being already so much weakened by the loss in this day of men and horses.'

But these are all of them mere empirical observations, mere happy remarks and verbal substitutions, which lead to unsurmountable difficulties when put to philosophical test.