Visibility/Invisibility Rule

Rule

Invisibility Rule states that two fauceirs are mutually invisible if and only if they do not share fauceirs that can interact.

The 'if and only if' indicates that this rule is reversible.

Visibility Rule holds that fauceirs may interact if and only if they contain common elements.

All fauceirs consist element fauceirs which again are formed of other more primordial elements.
These elements are capable of interacting at each level. However this interaction is unseen/unperceived by the main fauceir if the main fauceir 'deliberately' evolved fauceirs to suppress disturbances made by these interactions.

In physics we are aware of particles that cannot be detected by our present sensory devices because they don't interact with the type of matter all our devices are build from. Neutrinos are such particles known for a long time that travel through earth almost undisturbed. The Higgs-Boson, a particle that recently CERN scientists claim to have been discovered is another example.
In chemistry we know about so called inert substances such as glass that may refuse to react with strong substances such as HCl.
In biology certain hormones are visible to cells only that harbor the appropriate receptors.
In ecology such a phenomenon may evolve in viruses, for instance, to evade the defense system.
In psychology one brain cannot communicate with an other one if these brains do not contain a common denominator such as language or literacy.