In his seminal 1971 essay Freedom of the Will and the Concept of a Person, Harry Frankfurt provides an insightful account of free will and the problem of determinism, that is not metaphysical, but rather psychological and practical. First and foremost, though, Frankfurt offers a philosophical analysis of the nature of personhood. He does so by providing an alternative to the account of personhood advanced by Peter Strawson. In the Strawsonian view, a person is defined exclusively as a subject having both physical and mental properties, so that the concept of a person can simply be equated with the concept of a human being. Frankfurt argues against this view, assuming that mere membership in the biological species homo sapiens is not sufficient enough to decide on the question of personhood. Instead, he claims that the essence of a person is to be found in the structure of the will.