Ataraxia (Ἀταραξία "tranquility") is a Greek term used by Pyrrho and Epicurus for a lucid state, characterized by freedom from worry or any other preoccupation. [...] For the Pyrrhonians, owing to one's inability to say which sense impressions are true and which ones are false, it is the quietude that arises from suspending judgment on dogmatic beliefs or anything non-evident and continuing to inquire. The experience was said to have fallen on the painter Apelles who was trying to paint the foamy saliva of a horse. He was so unsuccessful that, in a rage, he gave up and threw the sponge he was cleaning his brushes with at the medium, thus producing the effect of the horse's foam.This sent me off looking for a passage I seemed to remember from somewhere about the spittle of horses (actually cows) threading the wind, which led serendipitously to a good definition of "foam(v.)" in Dyche's New General English Dictionary, Peculiarly Calculated for the USE and IMPROVEMENT of such as are unacquainted with the LEARNED LANGUAGES:
FOAM (v.) to be vastly enraged, angry, or mad, so that the spittle is as it were dried up, and comes out of the mouth involuntarily, like a wild boar that is closely hunted, and wounded; also to rise in froth or white scum, like a turbulent or disturbed sea.