The Causitive Factor
This can be the same for the people who we treat.  A person can come to see us with a myriad of symptoms that are all screaming loudly. 

Like a screaming child these symptoms may distract us from the underlying cause of the person's distress. 
Determining the correct CF, then, is a crucial part of Classical Five Element Acupuncture. The skill of the acupuncturist is through thorough questioning, listening and observing ascertaining which of the symptoms is primary and which are secondary, which are coming from the mother and which are coming from the child, which element is the causative factor?

The diagnostic foundation of Classical Five Element Acupuncture is the idea that each one of us has one Causative Factor or C.F. Element.
The Causative Factor is the one Element that is the foundation of the imbalances a person may be experiencing.  Once diagnosed, treating the C.F. Element then becomes the focal point for a Classical 5 Element treatment.
For example lets look at a patient who's C.F. is Metal.  The Metal Element is weakened - next via the Sheng cycle the Water element will also be distressed and show symptoms as it struggles from
the weakened preceding element. 
The Law of Mother-Child  

This law explains that if we walk into a room and there is a mother and a screaming child.

It is better to treat the mother who can sooth the child, instead of just treating the child - who may become upset again quickly. 
How to diagnose a persons C.F.
Even though we may do what we can to relive a persons immediate distress we can fall into the trap of just chasing symptoms as they appear, never finding the cause of any of them.
In time, following the Sheng and Ko cycles, this lack of flow continues around the cycle until all the five elements are struggling, the patient is in distress and arrives at the clinic with his or her list of symptoms. 

Instead of chasing the list of symptoms coming from the distressed elements, a 5 Element Acupuncturist goes right for the heart of the problem - the CF - which in this case is Metal.