Euclid wrote The Elements . A proof is an argument that something is true. Euclid required that proofs start from things that were accepted as true and proceed step by step to the thing being proved. Of course the logic underlying the deductions has to be accepted also. Two of Euclid's postulates for geometry were Postulate 4 .
All right angles are equal.
Postulate 5 . If a straight line falling on two straight lines make the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which the angles are less than two right angles. These two postulates say what is true . Euclid's other three postulates say what can be done Postulate 1 . To draw a straight line from any point to any point.