The primordial Bagua （eight trigrams） diagram derived from the eight combinations formed with three lines of

yin

-symbols and/or

yang

-symbols， as the hexagrams， with the

yin

-

yang

variation regulations of the hexagrams as the basis. The symbolic representations of primordial Bagua diagram， of binary numerical system， and of odd-even numbers to

yin

-

yang

and of all their interrelations were discussed. Similarly， the relationship between the primordial Bagua diagram and

Hetu

（《河图》） and

Luoshu

（《洛书》） were d

iscussed. The logic of the primordial Bagua diagram and

Hetu

Luoshu

is rigorous， reflecting the pattern of change of the solar irradiance and surface atmosphere temperature in the daily and yearly cycles， and they can be derived from each other， which explains the unity of the theoretical foundations of Chinese culture， including traditional Chinese medicine， at the root. In addition， the evolution process of the primordial to the manifested Bagua diagram was analyzed from two aspects， including concrete multi-factoral analysis and abstract coordinate rotation transformation. This paper also discussed interrelations involving the

Hetu

and

Luoshu

and the five phases， and also provided the unifying explanation for the following concepts of "the earth governs four seasons"， "the earth governs four extremities"， "the earth governs long summer".