The notion of transcendental observer (transcendentaler Zuschauer) is introduced in Husserl’s Cartesianische Meditationen. The idea of the observer first appears in Kant’s Copernican revolution putting the emphasis on the observer as he announces in his Kritik der reinen Vernunft. Contemporary physics also has a notion of a local observer and it plays a central role in the foundations of quantum mechanics, general relativity and cosmology.