So far in part 1  and part 2, we looked at how natural law is usually defined and how some of these definitions cause confusion. I also offered a definition that has always worked well for me. It helps to repeat it: Natural law is the law that says that all things work best or yield the best results when used according to their nature. As you can appreciate, this definition is not relative, but absolute. If you want the best tomatoes, you have to use the tomato plant according to its nature. If you don’t, you may still have pretty good tomatoes, but there will be no guarantee that the plant will yield the best results. The only way to guarantee that the plant will work best or yield the best results is to use it according to its nature. The key word here is the word “guarantee”. Natural law applies to everything even morality. If you put any question you may have about morality to the “natural law” test I believe that you understand all Catholic moral teaching. Since the current synod of bishops is about marriage and family, let’s look at how natural law can apply to our sexuality. Sex works best or yields the best results when used according to its nature (or if you will, according to its design). So, how do we know the nature or design of sex? We look at the circumstances under which it is guaranteed to work best or yields the best results. We could spend the next three days figuring this out, by exploring each and every sexual scenario to see which would yield the best results. But this work has already been done by St. Pope John Paul II in his Theology of the Body, so I am going to save you the time and trouble. (I’m sorry, but if you want to do the work, ask the right questions and be honest with your answers, you’ll come up with the same results.) The sexual act is guaranteed to work best and yield the best results when it takes place in the context of a relationship that is free, faithful, fruitful and total. This type of relationship is called marriage. Marriage is the union within which the sexual act is guaranteed to be free, faithful, fruitful and total. That, we can say, is also the nature of marriage. It is under those circumstances (when marriage is free and faithful and fruitful and total) that it is guaranteed to work best and yield the best results. This means that when marriage (and therefore the sexual act) is not free, faithful, fruitful and total, it may work – it may even be good – but it is not guaranteed to work best and yield the best results. So, forced or coerced marriages (not free), or marriages that take place under duress (as in during a pregnancy) are not guaranteed to work best or yield the best results. Marriages where the couple does not commit to an exclusive relationship, as long as they both shall live (not faithful) are not guaranteed to work best or yield the best results. Marriages that are not fruitful – and this does not only mean they have to be fertile – but that the love has to bear fruit; it must lead to good things, always, because love always makes us better and leads to good things – are not guaranteed to work best or yield the best results. As to the fullest expression of the fruitfulness of the sexual act in marriage, which is that it is pro-creative, it doesn’t mean that couples have to be fertile. It means that in its nature, the sexual act has to be fertile. So an infertile couple is still participating in a sexual act according to its nature. A sexual act that in its nature is not fertile is not using the sexual act according to its nature. Lastly, a marriage where the couple does not commit to loving each other totally, does not commit to total giving, is most certainly not guaranteed to work best or yield the best results. This is why we refer to marriage as a covenant and not merely a union or arrangement. This is why marriage is not just “two people who love each other, living together.” A marriage has to be free, faithful, fruitful and total. That means that the couple has to love each other freely, faithfully, fruitfully and totally. It means that the sexual act has to be free, faithful, fruitful and total. It means that the couple has to give themselves to each other freely, faithfully, fruitfully and totally. If the couple cannot (or refuses to) give of themselves to each other totally, then it’s not a marriage. It may be love, but it is not a marriage, because in its nature, marriage is total. That is the only way that it is guaranteed to work best or yield the best results. That is why the Church teaches that any use of the sexual act, outside of the context of marriage is immoral. The reverse is also true: When sex is not used according to its nature or design, it is not guaranteed to work best or yield the best results. I don’t need to explain all the problems that people have because of our sexuality not being used according to its design (as described above). The difficulty that we have with this is that most marriages are good or just ok. Some marriages are very good, but are they working best or yielding the best results? Can I love my wife freely, faithfully, fruitfully and totally all the time? Can I give myself totally – my whole being, my emotions, my fears, my sexuality, my body, my intellect, my fertility, my baggage, my dreams – to her all the time? Can she receive me totally and in turn give of herself totally to me? Probably not, but we sure can try. That’s what makes a marriage. That’s what we strive for. That’s what we commit to every morning when we wake up and we live our marriage vows on a daily basis. This should be a standard for how we live all our lives. We don’t settle for the least common denominator; we strive for God’s design in every aspect of our lives! Once we figure out through the natural law test, what the nature of marriage and sexuality is, we can strive to live it. Don’t you want to live your marriage the way God designed marriage to be? That is certainly guaranteed to work best and yield the best results! I truly believe that if we ask the right questions and we are honest with the answers, we can put anything to the “natural law” test and understand all Catholic moral teaching. If I was at the synod and was asked to make a presentation about natural law, this is what I would say. What would you say? Let me know your thoughts on this.