We do not relate to the world as such but through the interpretation of our perceptions and of our experience of the world. In this paper the work of Maturana, von Foerster, Bateson and Prigogine is examined from a biological and systems theory perspective, to provide support for the thesis that our knowledge of the world is a construction by an observer. Some conclusions relevant to therapy, as an outcome of the constructivist position, are also examined.