The Stoics similarly to Aristotle believed that everything in the Universe was ordered, but that it made no sense to seek happiness as the goal of life, because things do not always turn out as we plan them, and due to this we may become disillusioned. The Stoics believed that the natural way to respond to this sense of universal order was to act with honesty, aware of what part we play within the universe. The Stoics saw that the nature of a thing as its internal principle of change. So for them, an acorn would grown into an oak tree, given the right circumstances, because it has an inner principle which directs its growth.
Aquinas took on the thoughts of the Stoics and Aristotle. He agreed with a lot of what Aristotle said, but unlike Aristotle, Aquinas believed in God. Aquinas also believed that everything has a final cause and that if we, as humans, know what part we play in the Universe, and then we can figure out what out final cause is. Aquinas also came up with the primary and secondary precepts. The five precepts he came up with are: self-preservation and the preservation of the innocent, continuation of the species through reproduction, education of children, living in a society, and worshipping God. Aquinas believed that in order to live in accordance with nature, we must follow the primary and secondary precepts. In general everyone follows at least all four of them, as some may not believe in God. For example humans have sex for a primary precept of procreation and a secondary precept of pleasure and strengthening the relationship. Aquinas deduces the primary precept from the secondary precept. Therefore homosexual sex is fulfilling the second precept and it denies the primary precept. Although it is fulfilling one of the precepts, it is not really upholding the precept of procreation; therefore humans that do not fulfil the precepts do not live in accordance with their own nature.
Natural Law claims that we should use our reason, something that comes to us naturally, to examine the place and purpose that everything has. Natural Law claims that we should use our reason to examine the place and purpose that everything has within the Universe, and act in a way that reflects that understanding. It is not a utilitarian assessing of results or pleasing a majority, it is not based on intuitions or emotions, but on reason.
A moral life is a life lived according to and in accordance with reason. “To disparage the dictate of reason is equivalent to condemning the command of God.” From this Aquinas gathered that humans should do good and avoid evil, in accordance with their nature. Through this we can see that living in accordance with our nature, also means that we live morally. Therefore in a way the word ‘natural’ in Natural Law can also mean moral, because by fulfilling out final causes, we are living a ‘good’ life, or in other words a moral life.