The author considers the discovery of "mirror-neurons" and the individuation of a innate function in man, "the embodied simulation", very important. However, he thinks that there is a wider innate function, the "embodied function of adaptation", which is a system above the "embodied simulation" one. The author thinks that the adaptation develops through processes of simulation, syntonization, assimilation and accommodation. When two persons start interacting, since they are both provided with the "innate function of adaptation", they start up a complex Process of Reciprocal Adjustment (PRA). Relationship reachs a definition through a process of reciprocal adjustment in which the interacting persons select points of compatibility: attachment styles, MOI, family models, needs, etc. The isomorphism is a response that comes from the PRA during the course of treatment. The isomorphic behavior therapists are regarded as a powerful diagnostic tool and therapeutic. If the isomorphism is something that emerges from the encounter between two systems (patients system and therapists system), both equipped with adaptive function incarnate, the treatment system is something more and different than the simple addition of its parts. The "more and different", or the treatment system, is already a condition of change.
Keywords: Isomorphism, resonance, adjustment, empathy, co-evolution, mirror neurons.