Eyes and ears are bad witnesses to people if they have barbarian souls. Fragment B107.

In this pithy statement Heraclitus provides one of his two vivid metaphors meant to illustrate the fundamental importance of understanding the logos. Without an understanding of the logos, or the divine law or plan within nature, observation is absolutely useless. To try to investigate nature without understanding the logos, is like trying to gain meaning from words without understanding the language to which they belong: all that you perceive, in both these cases, is meaningless noise. (A barbarian is literally just a person who does not understand Greek).