For those who have studied modes before chances are good that you studied medieval modes. But a man named Boethius in the 6th century A.D. wrote down the Greek modes so now we have two kinds of modes. We use the medieval modes more. The Greek modes are considered obsolete. Remember that the Greek modes are theories not practices and that Greek music was highly improvisatory.
Theories are kind of like rules and practices means what people actually do.
Just like in life, in music we don't always follow the rules. Sometimes when several of us sing together or when we play an instrument with many chords like a piano, we seem to hear several notes that sound like one note. The Greeks played a part in the discovery and study of harmonics. They also felt that music was related to the universe and created by the cosmos. If you think it's anything like the kind of music that's around now, then you're wrong.