The next level of human character development involves not thinking of others as people to influence or be influenced by.
This is most easily understood in the dynamic of a marriage.
Now the marriage does not occupy any space. An atheist could say that marriage does not exist because he has never seen a physical phenomenon called “marriage”.
But it does exist, emerging from the will of the husband and the wife.
The Torah speaks of a husband and a wife becoming one flesh. In this sense, the child is the actual biological manifestation of the union between two human beings. But that union takes place even when the child does not.
This is why the trait of yesod is related to the thoughtful and unselfish relationship between a man and a woman. But it should also be seen in a broader sense to refer to the relationship between any two entities. Any two people. Even between man and God. This is why Kabbala and Chassidic writings see the definition of a “Tzaddik” as a function of this trait.
It means foundation, and it is the foundation of society, because it is the beginning of the understanding that “man is not an island.”
One thinks of a spouse doing something nice, not for herself, not for him, but for the marriage itself.