The New Natural Law (NNL) theory is the name given to a particular revival and development of Thomistic natural law theory, first proposed in the 1960s by Germain Grisez in an interpretative article on St Thomas Aquinas, in which Grisez challenged the then-dominant interpretation of Aquinas on natural law. In subsequent decades Grisez, John Finnis, Joseph Boyle and others richly developed the theory and applied it to other issues (free choice, moral absolutes, abortion, euthanasia, marriage and others).
According to the interpretation of Aquinas’ natural law theory standard in the 1960s, the basic moral criterion is human nature itself. Actions are morally permissible if they conform to the teleologies inscribed within human nature, morally wrong if not. Since the standard for what is morally right is what fulfils human nature, one must first determine by a theoretical procedure what end or ends fulfil human nature.