Some, such as Ernst Mally,[8] Edward Zalta[9] and arguably, Plato in his Theory of Forms,[9] have held that abstract objects constitute the defining subject matter of metaphysics or philosophical inquiry more broadly.
[11] Gottlob Frege said that abstract objects, such as propositions, were members of a third realm,[12] different from the external world or from internal consciousness.
Another popular proposal for drawing the abstract–concrete distinction contends that an object is abstract if it lacks causal power.
[citation needed] Quasi-abstract objects have drawn particular attention in the area of social ontology and documentality.
Some argue that the over-adherence to the platonist duality of the concrete and the abstract has led to a large category of social objects having been overlooked or rejected as nonexistent because they exhibit characteristics that the traditional duality between concrete and abstract regards as incompatible.
[14] These characteristics are exhibited by a number of social objects, including states of the international legal system.
[15] Jean Piaget uses the terms "concrete" and "formal" to describe two different types of learning.