In ontology and the philosophy of mind, a non-physical entity is a spirit or being that exists outside 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.
Main article: Abstract and concrete
See also: Abstraction
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 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.
If minds are fundamentally non-physical and non-local, we might expect that organisms without a central nervous system would show cognitive behaviors. Slime mold has no central nervous system, but has been shown in a laboratory environment to displays cognitive behaviors: “These results strongly suggest that, like humans, Physarum doesn’t attach any intrinsic value to the options that are available to it. Instead, it compares its alternatives.”
"What that means, according to Dussutour [...] is that a slime mold can learn — and it can keep that knowledge during dormancy, despite the extensive physical and biochemical changes in the cells that accompany that transformation. Being able to remember where to find food is a useful skill for a slime mold to have in the wild, because its environment can be treacherous. [...] More fundamentally, she said, this result also means that there is such a thing as ‘primitive cognition,’ a form of cognition that is not restricted to organisms with a brain.”
We might expect that if mind is fundamentally non-physical and non-local, that even organisms as small as single cells might have cognitive behaviors without relying on complex brain structures or even a central nervous system.
“For the first time, scientists have demonstrated that an organism devoid of a nervous system is capable of learning. A team from the Centre de Recherches sur la Cognition Animale (CNRS/Université Toulouse III -- Paul Sabatier) has succeeded in showing that a single-celled organism, the protist Physarum polycephalum, is capable of a type of learning called habituation. This discovery throws light on the origins of learning ability during evolution, even before the appearance of a nervous system and brain.”
“Associative memory is the main type of learning by which complex organisms endowed with evolved nervous systems respond efficiently to certain environmental stimuli. It has been found in different multicellular species, from cephalopods to humans, but never in individual cells. Here we describe a motility pattern consistent with associative conditioned behavior in the microorganism Amoeba proteus.”
We might expect that if minds are non-physical and non-local, that they could come (or should come) from somewhere else before birth rather than arising in the womb for the first time. Research has been done on this topic.
“Stevenson, an expert on psychosomatic medicine, suspected strong emotions are related to a child’s retention of past-life memories. Traumatic deaths, he thought, leave an emotional imprint. Indeed, most of the children he studied claimed that they had met a violent end previously. There was also a gap of a few years between lives; reincarnation is never immediate. And for the most part, souls seemed to stay local. That's to say, the ‘previous personality’ often lived in a distant village, but not quite so far away as to require a passport.”
"For the past 20 years, Dr. Jim Tucker, now the director of the Division of Perceptual Studies, has focused mainly on cases found in the United States. His most recent book Return to Life offers accounts of very strong American cases of young children who remember previous lives. In this book, Dr. Tucker writes about the now well-known cases of James Leininger, a young boy who had verifiable past-life memories of being a WWII pilot, and Ryan Hammond, who had verifiable memories of being a Hollywood extra and talent agent.”
We might expect that if minds are non-physical and non-local, that they could acquire knowledge non-physically and non-locally when the brain has completely shut off and has no access to the five senses.
Dr. Parnia & Patients with no brain activity: “Conscious awareness appears to have continued for up to three minutes into the period when the heart wasn’t beating, even though the brain typically shuts down within 20-30 seconds after the heart has stopped. This is significant since it has often been assumed that experiences in relation to death are likely hallucinations or illusions, occurring either before the heart stops or after the heart has been successfully restarted, but not an experience corresponding with ‘real’ events when the heart isn’t beating. Furthermore, the detailed recollections of visual awareness in this case were consistent with verified events.”
“There are four lines of evidence to explore [which support the mind being non-physical]. Number one is the unexplained recovery of consciousness for people who’ve been unconscious for prolonged periods of time moments or days before their death. Number two is a complex consciousness in people with minimal brain tissue. Number three is surprisingly complex consciousness in near-death experiencers when the brain is not functioning or functions at a diminished level, and number four; young children who recall details of a past life.”
We might expect that if minds are non-physical and non-local, that they could acquire knowledge non-physically and non-locally, under certain conditions.
“All but one of the nine experiments confirmed the hypothesis that psi exists. The odds against the combined results being due to chance or statistical flukes are about 74 billion to 1, according to Bem.”
See also: Dualism (philosophy of mind)
Dualism is the division of two contrasted or opposed aspects. 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. Pierre Gassendi put one such problem directly to René Descartes in 1641, in response to Descartes's Meditations:
[It] still remains to be explained how that union and apparent intermingling [of mind and body …] can be found in you, if you are incorporeal, unextended and indivisible […]. How, at least, can you be united with the brain, or some minute part in it, which (as has been said) must yet have some magnitude or extension, however small it be ? If you are wholly without parts how can you mix or appear to mix with its minute subdivisions ? For there is no mixture unless each of the things to be mixed has parts that can mix with one another.
Descartes' response to Gassendi, and to Princess Elizabeth who asked him similar questions in 1643, is generally considered nowadays to be lacking, because it did not address what is known in the philosophy of mind as the interaction problem. This is a problem for non-physical entities as posited by dualism: by what mechanism, exactly, do they interact with physical entities, and how can they do so? Interaction with physical systems requires physical properties which a non-physical entity does not possess.
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.
Other questions about the non-physical which dualism has not answered include such questions as how many minds each person can have, which is not an issue for physicalism which can simply declare one-mind-per-person almost by definition; and whether non-physical entities such as minds and souls are simple or compound, and if the latter, what "stuff" the compounds are made from.
See also: Spirit
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. Were the existence of ghosts ever demonstrated beyond doubt, it has been claimed that would actually place them in the category of physical entities.
Purported non-mental non-physical entities include things such as gods, angels, demons, and ghosts. Lacking demonstrations of their existence, their existences and natures are widely debated, independently of the philosophy of mind.