Lists of statements like this in mathematics are called axioms . Every mathematical sub-discipline has its own collection of common notions, definitions, and axioms, from which one deduces and proves the theorems of that discipline. There is a straight line joining any two points. Any straight line segment can be extended indefinitely. A circle can be constructed when its centre and a point on it are given.
All right angles are equal.
If a straight line falling on two straight lines makes the interior angles on the same side less than two right angles, then the two straight lines, if extended indefinitely, will intersect on that side. In school one encounters several theorems in geometry. Here I will list the ones I think are the most fundamental. Being theorems in geometry, all of these can be proven from the above axioms. Opposing angles formed by intersecting lines are equal.