Non-physical entity

In ontology and the philosophy of mind, a non-physical entity is a spirit or being that exists outside of physical reality. Their existence divides the philosophical school of physicalism from the schools of idealism and dualism; with the latter schools holding that they can exist and the former holding that they cannot. If one posits that non-physical entities can exist, there exist further debates as to their inherent natures and their position relative to physical entities.

Abstract concepts

Philosophers generally do agree on the existence of abstract objects. These include concepts such as numbers, mathematical sets and functions, and philosophical relations and properties. Such entities are not physical inasmuch as they exist outside of space and time. An abstract property such as redness also has no location in space-time. There are two concepts one being abstract and the other being concrete. The reason why these concepts are put into two separate categories is to make a distinction between metaphysics and epistemology. While older Cartesian dualists held the existence of non-physical minds, more limited forms of dualism propounded by 20th and 21st century philosophers (such as property dualism) hold merely the existence of non-physical properties.