Disgust is an emotion that is typically associate with things that are perceived as unclean, inedible, or infectious. In The Expression of the Emotions in Man and Animals, Charles Darwin wrote that disgust refers to something revolting. Primarily in relation to the sense of taste, as actually perceived or vividly imagined; and secondarily to anything which causes a similar feeling, through the sense of smell, touch, and even of eyesight. Disgust is one of the basic emotions of Robert Plutchik's theory of emotions. Disgust invokes a characteristic facial expression, one of Paul Ekman's six universal facial expressions of emotion. It is also associated with a fall in heart rate, in contrast, for example, to fear or anger.