The concept of quantity presupposes the concept of identical entities, which do not exist in nature (i.e., having all of the same properties, since at least one spatial dimension must differ to perceive discreteness). So, quantity must be a purely abstract concept, being a property of nature, but not part of it directly. This also presupposes that a purely abstract concept can realistically or accurately represent an entity. Obviously, it can do so accurately enough to help us progress both scientifically and technologically, but at what cost if we fail to understand something so fundamental?