The term person is used in common sense to mean an individual human being. But in the fields of law, philosophy, medicine, and others, it means the presence of certain characteristics that grant a certain legal, ethical, or moral standing.
For example, in many jurisdictions, the law allows a group of human beings to act together as a single composite entity called a corporation, and the corporation is considered a legal person with standing to sue or be sued in court. In philosophy and medicine, person may mean only humans who are capable of certain kinds of thought, and thus exclude embryos, early fetuses, or adults with certain types of brain damage.
Person and Personhood in relation to the human being are the concepts that were formulated in the early Christian theological tradition, during the first centuries A.D. by the great Church Fathers. The very concept of person (prosopon in Greek) was the result of a theological dispute, how God, according to the Christian (Orthodox) teaching, can be One and three at the same time. Further explication of the problem let to the formulation that there is one substance (or being) and three persons (hypostases): God Father, God Son and God Holy Spirit, but still just one God, not three. This theological concept of the person as something that has a specific identity and holds the fullness of being, was applied to the human being as well. The Church Fathers interpreted the "icon of God" in man as human ability to exist as a person, having his/her own unique identity in communion with other persons. Later in the West the concept was translated into Latin as persona and was explained by Boethius and St. Augustine as something characterized by rational capacities.
As an application of Social Psychology. and other discplines, phenomena such as the perception and attribution of personhood have been scientifically studied. Typical questions addressed in Social Psychology are the accuracy of attribution, processes of perception and the formation of bias. Various other scientific/medical disciplines address the myriad of issues in the development of personality.
In addition speculatively, there are three other likely categories of beings where personhood might be at issue:
Such questions are used by philosophers to clarify thinking concerning what it means to be human, or living, or a person.
The 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 the basic rights of liberty, freedom from persecution, and humanitarian care.
Since modern movements emerged to oppose animal cruelty (and advocate vegan philosophy) 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 exceptionalism (also referred to by its critics 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 many countries 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-people) or had awareness, but could never have awareness again due to massive and irrecoverable brain damage (some would call these post-people). The Vatican has recently been advancing a human exceptionalist understanding of personhood theory, while other communities, such as Christian Evangelicals in the U.S. have sometimes rejected the personhood theory as biased against human exceptionalism. Of course, many religious communities (of many traditions) view the other versions of the personhood theory perfectly compatible with their faith, as do the majority of modern Humanists.
The theoretical landscape of the 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 a person changed so much as to no longer be considered a member of the human species (by whatever standard is used to determine that).
The idea of extending personhood to all animals has the support of legal scholars such as Alan Dershowitz and Laurence Tribe of Harvard Law School, and animal law courses are now taught in 92 out of 180 law schools in the United States. On May 9, 2008, Columbia University Press will publish Animals as Persons: Essays on the Abolition of Animal Exploitation by Professor Gary L. Francione of Rutgers University School of Law, a collection of writings that summarizes his work to date and makes the case for non-human animals as persons.
There are also hypothetical persons, sentient non-human persons such as sentient extraterrestrial life and self aware machines. The novel and animated series Ghost in the Shell touch on the potential of inorganic sentience, while classical works of fiction and fantasy regarding extraterrestrials have challenged people to reconsider long held traditional definitions.