Person, in the classic sense, refers to a living human being. However, in philosophy, there has been debate over the precise meaning and correct usage of the term, whether the classical definition should be expanded or in some cases reduced, and which constituent elements or criteria must exist to establish personhood.
 Are all persons human?
Firstly, there is the simple and traditional view that the common usage is the correct one: that "person" does indeed mean "human". However, this runs into the problem that the term "person" has a somewhat loaded meaning - we commonly believe that all and only persons have certain rights, for example, the right to life. Some would go so far as to say that all and only persons are sacred. However, we can imagine the hypothetical alien from another planet, who, despite not being human, nevertheless has every trait that we see as being essential for this protected status that elevates it above mere objects. Thus, many claim that the simple view implies a sort of arrogant speciesism. There are also religious views that attribute personhood to supernatural beings such as gods, angels, demons, elves, and so on. Similar ethical debates centre around the question of animal rights and artificial intelligence.
Some would argue that humans have an amount of hubris that could potentially prevent us from recognizing the personhood of other sentient species. Some argue that certain primates and cetaceans (particularly the most highly intelligent species, such as the Great Apes and dolphins and killer whales) possess enough of the commonly held criteria for personhood to be considered persons. Recently corvids (crow species) have been recognized as highly intelligent tool users and strategists, while parrots display more linguistic intelligence. (see also animal intelligence and animal cognition). In fact, it is possible to teach a gorilla sign language, as evidenced by the case of Koko, who has expressed pride and flattery at having been awarded a place in the Guinness Book of Records for this.
Some extend the list still further. In the future sentient programs and computers may even emerge, extending the argument beyond all biological realms. The issue of how we might discern whether artificial information processing systems are conscious will likely become a matter of important debate. Alan Turing first suggested that we give convincing machines or programs the benefit of the doubt, using his Turing test. However, even a simple chatbot can fool people for a time. The best evidence for consciousness or "sentience" would be the subjective reports of people undergoing gradual replacement of brain tissue with artificial processors (see cyborgs). For example, surgeons are already beginning to integrate artificial "neural prosthetics" in patients to repair some forms of brain damage. At some point such a person's cognitive system could require no biological elements. Because the process is gradual, the success or failure of the facilitation of phenomenal elements of conscious experience in a particular device could be reported by a person whom we are pragmatically warranted is considered conscious from the outset. At least in artificial systems designed in the same manner, we would be much more warranted in ascribing personhood than the low standards of the Turing test. Justification would become extremely high for ascribing personhood to any artificial cognitive system that either (1) became artificial through a gradual process of "cyborgization" (permitting subjective reports from a highly trusted source) or (2) was an artificial cognitive system whose design replicated a completely artificial stage of a cyborgization process that has been proven effective, in all relevant respects (e.g. not "personality," but the same type of hardware, organization, and operations).
An elaboration on this theme is sometimes called "uploading", though the term carries baggage. "Uploading" here refers to a theoretical transfer of a mind to an artificial environment. Uploading proposals tend to assume that the mind does not persist through time in a substantial way (usually theoretically grounded on "deep reductionist" arguments that there is no soul or "self") so making a "copy" of brain structures is - in terms of survival - just as good as preserving the original structure. Advocates also tend to believe that such a copy would have conscious experience rather than merely acting as a convincing automaton. Reservations aside though, there does not seem to be any intrinsic reason that, some day, the mind of a human subject could not gradually extend into a functional simulation running on an advanced supercomputer. Related to this idea are cyberpunk/postcyberpunk science fiction (especially Ghost in the Shell) and the Transhumanist movement.
 Are all humans persons?
There are certain challenges to the classical view regarding disputes over whether certain humans are persons. For example, in the abortion controversy, although the fetus is clearly of the human species, it is a matter of debate whether or not it should be considered a person. Some argue that an early stage fetus is not a person because of its lack of any form of consciousness, while some would argue that they are persons because they are human, or because they will develop into conscious beings. Also, in the case of a victim of severe brain damage who has no mental activity, many would state that the person has ceased to exist, leaving only an "empty shell."
 Possible criteria for personhood
The above points seem to indicate that there may be persons that are not human, and there may be humans that are not persons. For these reasons, many philosophers have tried to give a more precise definition, focusing on some trait or traits that all persons, real and hypothetical, must possess.
The most obvious such trait that individuals considered persons usually possess is a conscious mind, typically (but not necessarily) with plans, goals, desires, hopes, fears, and so on. These traits therefore form a natural set of criteria for personhood.
Despite this, these criteria are controversial. In particular, some have argued that these criteria fail to recognize babies as persons. Although they meet some of the criteria, such as some degree of consciousness, and the ability to feel pain, the mental abilities of a newborn baby often seem to some to be no more impressive than many animals not commonly considered persons. Another problematic example is the status of a person in deep sleep, with no consciousness at the moment, but who upon waking would return to being an entity with full subjective awareness in the future. However, this latter case becomes less problematic with the assistance of theories of embodied subjectivity (mind-brain identity or unity), which allow for the persistence of an intelligent physical system that both has been self-aware in the past and has the capacity to continue being self-aware in the future. A variation of this example is a "reversibly comatose patient," though criteria for reversibility complicate such an analysis.
Because of these problems, some philosophers suggest that the potential to become fully thinking beings is sufficient to convey personhood, regardless of present mental status. A consequence of this view is that an embryo would be considered a person from conception; but others see the idea of a single cell - with absolutely no mind of any sort - being a person as counterintuitive. It is a matter of debate when in development any conscious awareness is possible, as seemingly cognitive behaviour that may or may not be attributable to stimuli can be seen at multiple stages in a pregnancy.
Nevertheless, consistent correlative evidence enables us to rule out awareness in the early and middle stages of foetal development with a high degree of confidence. As is reported in the August 24, 2005 article in Clinical Review, Foetal Pain: A Systematic Multidisciplinary Review of the Evidence, at 29 weeks of development fetuses have mature somatosensory evoked potentials that indicate that pain signals travel above the spine, through the thalamus and to the somatosensory cortex; and at around 30 weeks of development the brain's EEG signals suggest the first signs of wakefulness. Wakefulness is a necessary condition for any awareness, including pain recognition, but is insufficient for awareness without a functional somatosensory cortex to recognize pain signals as such - which is lacking for people in permanently vegetative states. Because these two necessary conditions for consciousness do not occur before the 29th week of development, fetuses cannot be consciously aware (and therefore subjects of experience) before the 29th week.
Another view amongst scholars is that personhood is not all-or-nothing: there can be degrees of personhood, based on how close to a fully working mind the individual in question has. Thus, a typical adult is entirely a person, while a human permanently in a persistent vegetative state would not be considered a person at all. Partial personhood is tacitly recognized by law in most cultures as reflected by parental rights and obligations, and in legal treatment of minors, the mentally handicapped, and the comatose. However, other philosophers argue that the concept of an incomplete or partial person is dangerous, possibly leading to weakened protection for those not considered complete persons. Others would argue that we are all incomplete or "developing" persons regardless of our developmental state.
 Personhood theory
According to Boethius:
- Person is an individual substance of rational nature. As individual it is material, since matter supplies the principle of individuation. The soul is not person, only the composite is. Man alone is among the material beings person, he alone having a rational nature. He is the highest of the material beings, endowed with particular dignity and rights.
John Locke emphasized the idea of a living being that is conscious of itself as persisting over time (and hence able to have conscious preferences about its own future).
In recent years a kind of consensus among secular scholars has emerged, which might be referred to as "personhood theory". This is strongly influenced by Locke's approach. The criteria a person must have in personhood theory are one or more of the following:
- The ability to steer one's attention and action purposively,
- Self-awareness, self-bonded to objectivities (existing independently of the subject's perception of it),
- Self as longitudinal thematic identity, one's biographic identity.
Neo-Kantian philosophers over the last two decades have emphasized that conscious awareness requires both:
- The sensorial capacity to access an environment (and one's own body) in a way that offers the basic qualitative content for subjective experience.
- The intellectual capacity to conceptually interpret sensorial content as representing some thing to oneself.
Both of these capacities are required for a subject of experience, action, thought, or self-reflection to exist, at least in the physically embodied, world-accessing manner of humans (and presumably other intelligent animals). As Kant wrote:
Without sensibility no object would be given to us, and without understanding none would be thought. Thoughts without content are empty, intuitions without concepts are blind. (Critique of Pure Reason, A 51 = B 75).
For those who consider an embodied capacity for subjectivity as necessary for personhood, these abstract constraints are quite relevant to the personhood theory debate. Advocates of alternative positions, such as a biological species or potentiality criterion, would instead need to provide arguments against embodied subjectivity as a basis for personhood. For example, one might argue that property claims are made by immaterial minds on immature material bodies, though any claim as to the nature of such minds would be necessarily speculative and would typically involve an argument for Cartesian substance dualism (see "mind-body problem").
 Implications of the personhood debate
Personhood theory has become a pivotal issue in the interdisciplinary field of bioethics. While historically most humans did not enjoy full legal protection as "persons" (women, children, non-landowners, minorities, slaves, etc.), from the late 18th through the late 20th century being born as a member of the human species gradually became secular grounds for an appeal for basic rights of liberty, freedom from persecution, and humanitarian care.
Since modern movements emerged to oppose animal cruelty (and advocate vegetarian or vegan lifestyles) and theorists like Turing have recognized the possibility of artificial minds with human-level competence, the identification of personhood protections exclusively with human species membership has been challenged. On the other hand, some proponents of "human exceptionism" (also referred to disparagingly as "speciesism") have countered that we must institute a strict demarcation of personhood based on species membership in order to avoid the horrors of genocide (based on propaganda dehumanizing one or more ethnicities) or the injustices of forced sterilization (as occurred in the U.S. to people with low I.Q. scores and prisoners).
While the former advocates tend to be comfortable constraining personhood status within the human species based on basic capacities (e.g. excluding human stem cells, fetuses, and bodies that cannot recover awareness), the latter often wish to include all these forms of human bodies even if they have never had awareness (which some would call "pre-persons") or had awareness, but could never have awareness again due to massive and irrecoverable brain damage (some would call these "post-persons"). The Vatican has recently been advancing a human exceptionist understanding of personhood theory, while other communities such as Christian Evangelicals in the U.S. have sometimes rejected personhood theory as biased against human exceptionism. Of course, many religious communities (of many traditions) find the more politically "progressive" versions of personhood theory perfectly compatible with their faith, as do the majority of modern Humanists.
The theoretical landscape of personhood theory has been altered recently by controversy in the bioethics community concerning an emerging community of scholars, researchers and activists identifying with an explicitly Transhumanist position, which supports morphological freedom even if some people change so much as to no longer be considered members of the human species (whatever standard is used for this determination).
Francis Fukuyama first brought the transhumanist philosophy to the attention of the bioethics community in 2002 with his critical book, Our Posthuman Future, in which he presents a bioconservative view.
 Individual rights and responsibility
Closely related to the debate on the definition of personhood is the relationship between persons, individual rights, and ethical responsibility. Many philosophers would agree that all and only persons are expected to be ethically responsible, and that all persons deserve a varying degree of individual rights (see human rights). There is less consensus on whether only persons deserve individual rights and whether persons deserve greater individual rights than nonpersons. The rights of non-person animals are an example of contention on this issue (see animal rights).
 Corporations as persons
Largely separate from the discussion of "real" persons are considerations regarding artificial persons such as corporations and states. In Santa Clara County v. Southern Pacific Railroad Company the United States Supreme Court ruled that a corporation is considered a person for many legal purposes. Many question the wisdom of this; the philosopher John Ralston Saul said, "If you are a person before the law and Exxon or Ford is also a person, it is clear that the concept of democratic legitimacy lying with the individual has been mortally wounded." It must be emphasized that corporate personhood is a legal fiction -- in other terms, a convenient assumption adopted for practical reasons that is not necessarily accepted as true (see also the documentary film The Corporation).
 See also
- Neural Correlates of Consciousness (NCC)
- animal liberation