In the Fourth Way there is something known as “The Law of Three”. This “Law” relates to the three forces: Passive, Active and Neutral (or Allowing) force.
For example, consider throwing a rock. The Active Force is you throwing the rock, the Passive Force is the rock itself and The Allowing (or Neutral Force) which is the most difficult to perceive force, is the space in which the rock is able to be thrown.
If you took away any one of these forces then the rock would not be thrown. The Law of Three is depicted in the Yin, Yan, Yung symbol.

But it goes deeper than this. In the Fourth Way tradition three Centers are recognized: Moving/Instinctive, Emotive and Intellectual. These three are active within all human beings to a greater or lesser degree. Almost without exception, a person will be focused more within one of these than the others and for most people it is not difficult to determine which of the three they favor.
For instance people who are focused more on the Moving/Instinctive center will be largely concerned with food/sleep/exercise/dance/sports etc. Emotionally centered people are focused on Art and relationships. Intellectually centered people tend to read a lot and enjoy things that are related to symbology such as language, computer programming etc. Nobody is active in only one center but all tend to have one that dominates the other two.
The three centers also correspond to our three brains: Mammalian(Emotive), Reptilian(Instinctive) and Primate(Intellectual) brain. This follows how life on this planet evolved – Dinosaurs were primarily reptilian living a solo existence while Mammals tend to live in groups and need emotions in order to get along with each other. Primates(like us) poses the added ability to speak/understand a symbolic language.
The three centers correspond with three major Chakras: Muladara (moving/instinctive), Anahata (Emotive) and Ajna (Intellectual). Therefore these three Chakras correspond to our three brains: Reptilian (cerebellum), Mammalian (Limbic System – hippocampus, hypothalamus, amygdala), Primate(Neocortex).

If we are hoping to balance our three centers then it is vital to gain a solid understanding of them. The primary way to do this is with Self Observation but that will be the subject of another article.
There are many examples we can find in organized religion of the Law of Three, for example the Holy Trinity of Christianity(Father, Son and Holy Spirit) and even the Body, Mind and Spirit. No matter what you believe the Law of Three provides us with a framework that we can use to better understand ourselves and others. I see it as a fundamental truth and perhaps, The fundamental truth.