There are myriad approaches from various scholarly disciplines to respond to this fundamental question of human nature. Psychologists probe the contents of the conscious and subconscious mind to help individuals understand their authentic self. Sociologists observe how human beings behave collectively to determine if there are any informative generalizations that may be drawn. Anthropologists and biologists are concerned with how human beings have culturally and physically evolved over eons of time. Theologians of different religious traditions seek to define and justify certain beliefs about humanity''s place in the universe--whether, for example, each of us exists as a special creation in the "image and likeness of God" (imago Dei) or are merely a drop in the cosmic ocean of being with no individual essence. Ethicists debate the moral status of human beings at various stages of life and what specific rights and duties are applicable to, for example, embryos, fetuses, infants, children, cognitively disabled adults, irreversibly comatose patients, and the deceased. Finally, metaphysicians investigate, among others, the following interrelated questions: "What composes a human being?" or "With what is a human being identical?" and "What accounts for a human being''s persistence through time and change?" The first pair of questions is concerned with determining what material or immaterial substance or set of parts is necessary in order for a human being to exist--for example, a living body of the species Homo sapiens, a functioning human (or human-like) brain, a mind distinct from one''s body and brain, or a non-physical soul somehow related to one''s physical body. The last question pertains to what is necessary for one to continue existing as the numerically same human being despite physical and psychological changes we inevitably experience.

These questions regarding the ontology of human beings have been a central concern throughout the history of philosophy, with multiple accounts having emerged of what constitutes the essence of human nature--an area of inquiry sometimes termed "philosophical anthropology." The term "essence" refers to the set of specific parts, properties, capacities, etc. that are shared by all and only human beings. This is not to say that non-human entities may not also possess some of these essential human features, but possessing the entire set--whatever the set comprises--is both necessary and sufficient for one to count as a human being. In the West, the Greek philosophers Plato and Aristotle offered distinct views of what a human being fundamentally is. For Plato, a human being is identical to an immaterial soul--construed equivalently to what we would today call a "mind"--that is "imprisoned" for a time in a material body before death sets it free, either to be united with another body or to spend eternity contemplating the source of being, truth, and goodness. Aristotle conceived of a human being as a composite unity of an immaterial soul and a material body of which the soul is the formal principle--a view known as hylomorphism. This basic controversy regarding a human being''s relationship to her material body has continued to drive debate among philosophers throughout the ensuing centuries into the present day.

Numerous accounts have been proffered identifying the human essence as an immaterial soul or mind, a living animal body, a functioning brain, or a bundle of psychological states, to cite some of the principal views. Depending on which of these theses one favors, the criterion of a human being''s identity through time and change consists in sameness of soul or mind, continuity of biological life processes, continuity of neural functions, or some form of psychological continuity involving memory, personality traits, or self-consciousness. In contemporary analytic philosophy, the methodological school of thought in which the present investigation is situated, the debate between philosophers who reduce human nature to either its physical or psychological properties, those who hold that human nature includes both types of properties, and those who argue that human nature transcends such properties has focused on three distinct camps. Substance dualists maintain a contemporary version of Plato''s view that a human being is identical to an immaterial soul that is conjoined to a material body during one''s earthly life. Reductive materialists contend that human nature is nothing "over and above" the biological and neurophysiological facts which are subject to empirical scientific investigation: all the physical and psychological states of a human being can be wholly explained in virtue of the physical properties had by one''s body. Finally, non-reductive materialists take seriously the data provided by empirical science, while nevertheless maintaining that there are some aspects of human nature which cannot be wholly explained in terms of physical properties alone. The non-reductive thesis is not intended to imply that human nature includes an immaterial component that essentially exists with absolutely no reference to a physical body, as substance dualists claim. Rather, the thesis is that some states of a human being--namely, certain types of psychological states--cannot be explanatorily reduced to states of one''s physical body, such as neurons firing in the cerebrum; rather, a further psychological explanation is required.

(excerpted from chapter 8).