This book investigates the ontological state of relations in a unique way. Starting with the notion of system, it shows that the system can be understood as a relational structure, and that relations can be assessed within themselves, with no need to transform relations in elements. "Relations" are understood in contrast to "relational property": without a relation there is no identity, therefore no existence. What allows us to do that without hypostatizing the relation, and without immediately taking it simply as a causal relation, can be better grasped, possibly, in reference to a few entities that make best display of their systemic nature, for example images, works of art, and virtual bodies. This book shows how virtual bodies are ontological hybrids representing a type of entity that has never appeared in the world before. This entity becomes a phenomenon in interactivity and evades the dichotomy between "external" and "internal"; it is neither a cognitive product of the consciousness, nor an image of the mind. The user is well aware of experiencing another reality, also in the sense of a paradoxical reduplication of perceptual synthesis. The virtual body-environment is therefore simultaneously external and internal, with virtual bodies-environments to be seen as artificial windows to an intermediary world. In this intermediary world, the space itself is the result of interactivity; the world takes place in the sense or feeling of immersion experienced by the user; and the body, perceived as "other", takes upon itself the sense of its reality, of its effectiveness, as an imaginary and pathic incision, as a production of desire and emotion, to the point that the feeling of reality conveyed by a virtual environment will rely significantly on how this environment produces emotions in the users.