The sixth proposition is a converse of the fifth, proving that if the base angles are congruent, then their corresponding sides are congruent. This is Euclid's first proof by contradiction. A proof by contradiction proves a statement by assuming it is false, then showing how that leads to an impossible conclusion. Euclid assumes that the sides are not equal, such that AB is the greater side. Since AB is greater than AC, AC is placed onto BA resulting in D. By SAS congruence, this means that △DBC is congruent to △ACB. This is a contradiction, because the greater is equal to the less. Since assuming that one of the sides is greater than another led to an impossible statement, the assumption must be false, therefore the two sides are equal.