were supposed to be real; and then b must of course be real. but we supposed b to be impossible. let it be impossible then. if, then, b is impossible, a also must be so. but the first was supposed impossible; therefore the second also is impossible. if, then, a is possible, b also will be possible, if they were so related that if a,is real, b must be real. if, then, a and b being thus related, b is not possible on this condition, and b will not be related as was supposed. and if when a is possible, b must be possible, then if a is real, b 