The masticatory system is widely adaptable and tolerant especially in young people ; this characteristic can be helpful but it can also incur risks because there are limits to everything. Guided solely by the insubstantial support provided by data derived from evidence-based medicine, a researcher would have to be both excessively clever and unusually cunning to define the borderline between physiological and pathological occlusion. Accordingly, the optimization of masticatory function through the intermediary of efficient occlusal interaction of upper and lower teeth remains a fundamental common sense objective of all dental treatment. This common sense is based on the fundamental principles of simplicity and of ergonomy that govern all biological "mechanisms ". By respecting the principles of "centering, stabilization, and guidance", dentists will adhere to a simple but precise manual for preserving or creating, if need be, the good occlusal relationships that encourage unfettered mandibular movement (fitting) and symmetrical global functioning (centring), and absence of interference (guiding).