The essay is concerned with the topic of the difference between humans and animals in ancient Greek thought. Starting with the lexical problems in studying ancient terms indicating animals, the author presents various definitions of man formulated in contrast to animals, treated as inferior beings. She focuses on the term logos, understood both as reason and ability to speak, which the animals were deprived of according to most of Greek thinkers. The author shows also how the idea of man uniqueness has changed from the archaic period to the classical one. At the end of the essay several reasons why the ancient philosophers define man in opposition to animals are suggested.