Throughout the work the author pursues one objective: to show that intentionality belongs to the fabric of reality, hence it is also the source of the intelligibility of this reality. As such, it has an ontological status and a causality of its own which enables it to play its role as intermediary between the knowing subject and the object of thought. This interdependence is responsible for its nature and the two-way movement expressed by intentio intellectus and intentio rei whose efficacy widens or reduces the scope of what is considered as intelligible by a philosophical doctrine.