Definition
Describing in philosophical terms what a non-physical entity actually is (or would be) can prove problematic. A convenient example of what constitutes a non-physical entity is a ghost. Gilbert Ryle once labelled Cartesian Dualism as positing the "ghost in the machine". 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. It is not the supposed ability of ghosts to penetrate solid matter, as neutrinos can do that, and lack of mass is also exhibited by photons. Most tellingly, were the existence of ghosts ever be demonstrated beyond doubt, in terms of the physicalist/dualist philosophical debate (at least) that would actually place them in the category of physical entities.
This problem of definition is a problem for the physicalist school of philosophy. Without any definition of non-physical entities, physicalism would be a trivially true philosophical position that everyone could subscribe to, but that would thereby lack much in the way of significance. So physicalist philosophers usually defer to physics, and the natural sciences, for a definition of what is physical and what is non-physical.
Read more about this topic: Non-physical Entity
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