“Abraham” is the first spiritual property that we reveal within. It is the main quality of bestowal, Hafetz Hesed (one who wants mercy). When Abraham reaches “old age,” meaning that a certain stage is completely revealed to him, a new state emerges which is fundamentally opposite to the preceding stage.
This is a process in which Abraham continuously raises himself above his previous desire associated with Babylon before attaining the next stage. First, he distances himself from all the forces and desires of Babylon’s inhabitants who worship other gods. When he detaches himself from it, he leaves that place or stage, and a new stage emerges. On this new stage, the cycle repeats. Once again he reveals the desire within, which returns him to the previous state he just left.
Since he is continuously growing above desires qualitatively, it only seems like a new stage emerges. A desire reveals itself on the “left side” opposite to Abraham, yet, at the same time, it is his “son,” which means it is a consequence originating from Abraham. This is how "Isaac," which signifies the left line, Bina, Gevura (prevailing, overcoming), emerges as the source of all impure forces associated with the left side (Klipot).