Catholic theology of sexuality, like Catholic theology in general, is drawn from natural law, canonical scripture, divine revelation, and sacred tradition, as interpreted authoritatively by the magisterium of the Catholic Church. Sexual morality evaluates sexual behavior according to standards laid out by Catholic moral theology, and often provides general principles by which Catholics can evaluate whether specific actions meet these standards. The Church teaches that sexual intercourse has a two-fold unitive and procreative purpose; According to the Catechism of the Catholic Church, "conjugal love ... aims at a deeply personal unity, a unity that, beyond union in one flesh, leads to forming one heart and soul", since the marriage bond is to be a sign of the love between God and humanity. Because Catholics believe God found everything he created to be "very good", the Catholic Church teaches that the human body and sex must likewise be good. Every person is created in the image of God and therefore has great dignity including their sexuality. Sexuality is not something purely biological; rather, it concerns the intimate nucleus of the person. In cases in which sexual expression is sought outside marriage, or in which the procreative function of sexual expression within marriage is deliberately frustrated (e.g., the use of artificial contraception), the Catholic Church expresses its concern. According to the Catechism, among what are considered sins against chastity are masturbation, fornication, pornography, and homosexual practices. Additionally, "adultery, divorce, polygamy, and free union are grave offenses against the dignity of marriage". In Church history there have been significant differing opinions on the nature of the severity of various sexual sins. In the present Church likewise there exists still wide opinions by theologians and much of the laity on official Church teaching on sexuality. Natural law Natural law () refers to the use of reason to analyze human nature to deduce binding rules of moral behavior from God's creation of reality and mankind. "The natural law is written and engraved in the soul of each and every man, because it is human reason ordaining him to do good and forbidding him to sin." It is called "Natural", because the reason which decrees it properly belongs to human nature. Its main precepts are found in the Ten Commandments. In the Summa Theologica, St. Thomas Aquinas wrote: "...the rational creature is subject to Divine providence in the most excellent way, in so far as it