The tetractys, a “tetrad” made up of unequal members, is a cryptic formula, only comprehensible to the initiated…. It was spoken of as the answer to the question: What is the oracle of Delphi?
Edinger, following Burkert, identifies the tetractys-as-tetrad as the Pythagorean sacred formula encoding cosmic harmony and the music of the spheres, establishing the numinous foundation of the fourfold principle.
, The Psyche in Antiquity, Book One: Early Greek Philosophy From Thales to Plotinus, 1999thesis