This chapter explains the formal foundation of natural law, the law of a practical reason; one must remember that Saint Thomas also evaluated the treatment of natural law with a metaphysical approach, from the perspective of participation: natural law is a participation of eternal law. This metaphysical approach is very important to the main subject of natural law because of the reasons that Saint Thomas himself gives. Saint Thomas points out that natural law are not a habit of nature, but an 'opus rationis', understood as an 'operatum rationis'. The authors of the Golden Age, such as Vitoria, Soto, Báñez, Molina, Suárez and Araújo, have made it clear that natural law, in as much as it is in the human being. After Suárez's criticism of Vázquez, different philosophers from the Thomist school also made an effort to stress that in Vázquez's hypothesis natural law cannot be called the participation of eternal law, given by God as legislator.