In moral decision making, the Catholic perspective has traditionally relied on what is called Natural Law. The idea behind natural law is as follows. If God is rational and God created the universe, then God must have created a rational universe. A rational universe is one in which there is a logical (rational) order and everything has an end or purpose proper to it. Put another way, there is a particular reason for why each thing and being exists. This being the case, the best way for that thing to be used, or the best life for that being to live is the one for which it was created. For example, apples are good for the generation of new apple trees (via their seeds) and as food for animals. One could also say that they are beautiful. But apples aren't particularly good materials for building houses, and apples cannot be good elephants. Human beings are created in the image of God and so they too are rational beings. This capacity for rational thought means that human beings can work out the order of the universe and so can see, even without divine revelation, what the proper purposes or ends of different things and beings are. Most things tick over on their own, achieving the ends for which they were created and without much fuss. So, apples are eaten, their seeds grow into new trees and new apples are produced. Most things tick over on their own because most of the universe is created by God to function in a predictable way as described above. Human beings, however, are different, because they are free. We get to make choices about what we use things for, the ends or purposes to which we put things. In a similar way we get to make choices about what we strive for, the kind of people we want to be. It is because of this freedom that it makes any sense to talk about morality. The natural law corresponds to the rational order intended by God. We use our own rationality to discern this natural law, and then use our freedom to choose to direct our actions towards proper purposes, or ends, outcomes and goals in accordance with this natural law. If we use our rationality and our freedom in this way to discern the natural law then we are probably also acting in accordance with Divine Law. In other words, if we do what is the most rational thing in light of the different ends or purposes of things and beings, then we are most probably acting in accordance with God's will, a significant factor in our own purpose for existence. If we act in accordance with God's will, then we are realizing God's love in the world. In this way building the society of justice, peace and joy that God has promised us and participating in the flourishing that God wants for the whole of creation.

The Catholic perspective has inherited the idea of the natural law from the Greek philosophers. However, our contemporary knowledge about the natural world is significantly more sophisticated and scientifically grounded than either the Greek philosophers or even most of the 2000 years of Catholic intellectual history. Today we understand far more how apparently random or chance events have played a role in the creation of the universe, of Earth, of life, and the evolution of human beings. The natural law, however, is not to be confused with the 'law of nature'. The natural law does not claim that just because something exists in nature in a particular way then that is the way that something is meant to be. This is especially so when it comes to things that are influenced by human choices. Just because something is a particular way, doesn't mean that it should or ought to be that way. It was once thought that monarchies were divinely instituted ways of governing human societies and that being a king was a divine right. Most people did not question this 'order', because it seemed so 'natural'. We now know that there are many other, and often better, ways of governing people, ways that respect human dignity and contribute to human flourishing. Natural law plays a role in social progress because, through the use of our rationality and freedom, and love, we have seen that feudal societies seldom contributed to respect for the dignity of every human being and their flourishing. This is not to say that contemporary democracy fully achieves the social ideal either. But contemporary democracy represents progress towards the ideal because through the natural law, democracy has been able to cement, at least in law, the idea of natural and universal human rights that are due to all people by virtue of their being human. Another way of putting it is to say that the natural law is the good of the human person making moral demands on us. For this reason, a pastor cannot feel that it is enough simply to apply moral laws to those living in 'irregular' situations, as if they were stones to throw at people's lives. This would be- speak the closed heart of one used to hiding behind the Church's teachings, 'sitting on the chair of Moses and judging at times with superiority and superficiality difficult cases and wounded families'…. By thinking that everything is black and white, we sometimes close off the way of grace and of growth, and discourage paths of sanctification which give glory to God. Let us re- member that 'a small step, in the midst of great human limitations, can be more pleasing to God than a life which appears outwardly in order, but moves through the day without confronting great difficulties'. The practical pastoral care of ministers and of communities must not fail to embrace this reality (Pope Francis, 2016 Amoris Laetitia).

In what follows, we shall look at particular moral issues where the Catholic perspective has a specific stance that is derived from its application of the natural law (i.e., human reasoning about the proper purposes of things and beings) and revelation (i.e., what we know about what we ought to be aiming for, who we ought to want to become, and how we ought to get there based on the God's full revelation in Jesus Christ, and through the working of the Holy Spirit in the life of the Church).