I had something in mind like, assume that the last line is not a contradiction, then we have c( e- a) on the left and something smaller than one on the right. H

I had something in mind like, assume that the last line is not a contradiction, then we have c( e- a) on the left and something smaller than one on the right. However, assume the last line is not a contradiction is false, so I can't build the argument atop of it.