Apart from Einstein, Paul Dirac was probably the greatest theoretical physicist of the twentieth century. Dirac, co-inventor of the most revolutionary theory for 150 years 'quantum mechanics' is now best known for conceiving of anti-matter in his head and also for his deeply eccentric behaviour. For him, the most important attribute of a fundamental theory was its mathematical beauty, an idea that he said was 'almost a religion' to him. In this talk, Farmelo will argue that this obsession originated in his early life and training as an engineer and mathematician. An examination of Dirac's character will show why he was sometimes called 'the strangest man' in the modern history of physics.