According to the basic rules of Systems Theory, systems are both more and different than their constituting parts, and the rules governing a system overrule those of its parts.

All systems are part of a larger system while composed of smaller systems, and the various systems interact with each other on different levels.

Systems, therefore, cannot fully be understood from their parts. Neither can a part – e.g. an individual human being – be understood without the various internal and external systems which it is a part of.

Psychologically, we are a psycho-somatic system in which our body, mind, emotions, thought processes, behaviors, the different parts of our personality mutually influence and determine each other.
We live in an endlessly complex web of external (environment, society, etc.) and internal (body, mind, personality parts, etc.) systems – and our problems are also parts of these very same systems.

Since the emergence of system-based thinking around the middle of the last century, human behavior in general and our psychological problems in particular have also been approached from this distinctly new perspective.
Group, family and couples therapies tend to focus on our external, whereas individual therapies on our internal systems.
However, our internal and external systems are inseparable.

In the world of modern psychotherapy today, basically all modalities are founded on systemic principles.