Intentionality, as Brentano originally introduced the term in modern philosophy, was meant to provide a distinctive characteristic definitively separating the mental from the physical. Mental states have an intrinsic relationship to an object, to that which they are ‘about.’ Physical entities just are what they are, they cannot, by their very essence, refer to anything, they have no ‘outreach,’ as one might put it. Mental states have, as it were, an incomplete essence, they cannot exist at all unless they are completed by something other than themselves, their object. Brentano's position is opposed to all theories which represent the mental as only extrinsically related to the world, that is, to all theories in which mental states are themselves self-sufficient for their own existence and only secondarily relate to the world by means of something external to their nature, e.g., neurological causation, divine intervention, or preestablished harmony.