What was the reason for the ancients to promote the establishment of the first wife and the first son?

The root of this idea is the Taoist aristocracy's idea that "the son is valued by the mother", which is fundamentally different from the "mother is valued by the son" idea advocated by the Confucian commoners later on.

Why is it that "the son is valued by the mother"?

Because the Taoist way of looking at things is that "everything is negative and holds the yang", and the reproduction of human communities cannot be separated from the "belief in the mother goddess", and the management mode of human communities should be "mother in charge". The first is the "mother of all".

The leader of a country could only have one supreme wife. People at that time did not recognize the role of men in the act of procreation, and believed that women had the magical power of procreation, so the children she gave birth to were the first-born, the sons were the first-born, and the daughters were the first-born.

And a first wife often meant that the mother's family was quite powerful and could have strong foreign aid, and a first-born son as a leader often meant that the kingdom was already basically 80 percent secure.

The Tao Te Ching says, "The Tao begets one, the Tao begets two, the Tao begets two, the Tao begets three, and the Tao begets all things."

The common understanding of Taoism is that the way of development of human communities is first to produce the Taoist harem as "one", because "all things are negative yin and embrace the yang", she will cultivate the emperor as "two

The Taoist harem is the "one" that produces the Taoist harem, because "everything is negative yin and holds yang", she will nurture the "two" that is the emperor, and then the "two" that is the emperor will select the "three" that is the Confucianism that does the concrete work of the administration and the military that does the concrete work of the military and will finally control the functioning of everything under heaven.

The Taoist ideology is somewhat more "parliamentary" and mom-and-pop thinking.

The Legalist understanding is that the way of development of human communities is first to produce the emperor as the "one", who chooses the harem and powerful ministers as the "two" in accordance with the Taoist mode of thinking, and then they assist the emperor in selecting the "three" of Confucianism, which is engaged in administrative work, and the "three" of Military Science, which is engaged in handling military affairs. Then they assisted the emperor in selecting the "three" of Confucianism in administration and military affairs, and finally controlled the operation of everything under the sun.

The Legalists, on the other hand, favored a presidential system and a dad mentality.

In general, they are all patriarchal thinking, the primary goal is to consider the purity of the lineage, Qin Shi Huang saw this, so he did not set up a queen for life, that is, he did not set up a "first wife", which is a defect that made the Qin dynasty die in the second world, therefore, the subsequent dynasties have coincidentally set up a "first wife", which is the first wife of the Qin dynasty. "The first one is the first one, so as to avoid the situation of having no one to succeed them.