One of the central problems of metaphysics is what it is to be a person. The answer ought to account for central phenomena of personhood; rationality, command of language, self-consciousness, control or agency, and moral worth or title to respect, are amongst the salient characteristics that have been thought to distinguish persons from other forms of life. In Locke, ‘person’ is a forensic term, applying for moral reasons (‘to agents capable of a law, and happiness and misery’, Essay, ii. 27). A dualistic approach regards a person as an amalgam of an essentially separate mind and body, with the resulting problem of reinventing their unity in the living person (see mind-body problem, occasionalism). Monistic theories, such as that of Strawson’s Individuals (1959), work with a primitive concept of a person, as some one thing logically capable of being described in bodily or mental terms. A popular modern analogy is with the compatible software and hardware descriptions of a computer (see functionalism).