Such objects include concepts such as numbers, mathematical sets and functions, and philosophical relations and properties.
The dualist school supposes the existence of non-physical entities, the most widely discussed one being the mind, but beyond that it runs into stumbling blocks.
[7] Dualists either, like Descartes, avoid the problem by considering it impossible for a non-physical mind to conceive the relationship that it has with the physical, and so impossible to explain philosophically, or assert that the questioner has made the fundamental mistake of thinking that the distinction between the physical and the non-physical is such that it prevents each from affecting the other.
[9][10] However, it is hard to define in philosophical terms what it is, precisely, about a ghost that makes it a specifically non-physical, rather than a physical entity.
[10] Purported non-mental non-physical entities include things such as gods, angels, demons, and ghosts.