*he developed the theory of opposites, which concerned how people could be treated using the four humours
*Galen's work on anatomy was based on information gained by dissecting animals such as pigs and monkeys
*through this work, he discovered that the brain controls speech
*he also made mistakes: he believed that blood passed from one side of the heart to the other through tiny holes in the septum (the dividing wall between the left and right sides of the heart) when blood instead passes around the heart through veins and arteries. he also said that the human jaw bone is two separate bones, when it is actually one