The 18th Century was known as the 'Age of Reason'. It is ironic that one of the most famous people from that era is King George III (1738 – 1820) of the United Kingdom, who was mad. During his periods of insanity, the ruling was done by his son George, Prince Regent, giving us the term 'Regency' for this particular period.
There have been various attempted diagnoses of the madness. One of the most popular theories, although by no means proved, is that the king suffered from 'porphyria', a disease named after the colour purple, as it causes the urine and faeces to have that hue.