Geometry is the branch of mathematics dealing with shapes and sizes. Euclid was a Greek mathematician from around 300 BC who is best known for his influential textbook 'Elements', which laid out the foundations of geometry and introduced logical reasoning and mathematical proofs. The 'Elements' begins with plane geometry and is based on five postulates from which many other geometric properties can be deduced. Euclid was the first to show how geometric propositions could fit into a comprehensive deductive system.