Euclid's fifth postulate states that 'given any two lines and a transversal, if the angles on the same side of the transversal are less than 180 degrees they will meet on that side of the transversal'. The statement given here is that two intersecting lines cannot be perpendicular to the same line, i.e., parallel. This statement is stating that the co-interior angles of intersecting lines cannot be 90 degrees in measure, and it is almost equivalent to Euclid postulate 5.
