People who experience an immersive VR system usually report feeling as if they were really in the displayed virtual situation, and can often be observed behaving in accordance with that feeling, even though they know that they're not actually there. Researchers refer to this feeling as "presence" in virtual environments, yet the term has come to have many uses and meanings, all of which evolved from the notion of telepresence in teleoperator systems. In Presenccia, we take an operational approach to the presence concept. Our approach lets us assess the extent of presence using tools beyond traditional questionnaires, and therefore we avoid many of the problems involved with sole reliance on these. Instead, we consider the extent to which mixed reality (MR) and VR participants realistically respond to virtually generated sensory data. Specifically, we measure the similarity of their response with what we might observe or predict if the sensory data-the situation, place, or events-were real, rather than virtual. We consider this response on several levels.