Euclid's Elements
Euclid's fourth axiom says that all right angles are equal. But isn't that obvious? 

The famous mathematician Euclid is credited with being the first person to axiomatise the geometry of the world we live in  that is, to describe the geometric rules which govern it. Based on these axioms, he proved theorems  some of the earliest uses of proof in the history of mathematics. 

Starting in this issue, PASS Maths is pleased to present a series of articles about proof and logical reasoning. In this article we give a brief introduction to deductive reasoning and take a look at one of the earliest known examples of mathematical proof.
