Primary precepts derived from natural inclinations
Aquinas discusses the derivation of the primary precepts in 1-11 Q94 A2. The logic of his argument can be expressed diagrammatically.
SYNDERESIS --------------> INCLINATIONS--------------- > PRIMARY PRECEPTS
Man is a rational agent who seeks goods he believes perfect his nature. So the starting point is the synderesis rule: do good and avoid evil. The ends of man are of three sorts, argues Aquinas, which together we can form into the acronym POWER (preservation of life, ordered society, worship of God, education and reproduction).
1. Those we share with all creation (the desire to preserve our life).
2. Those we share with animals (reproduction and the teaching of our offspring).
3. Those that are uniquely human (living in society and worshipping God).
But the inclinations are only good in so far as they are subject to reason; they need to be humanised, made moral, pursued with deliberation, judgement, responsibility. So the primary precepts give us general ends which we need to apply to our real world circumstances using the virtue of practical wisdom, in order that we (and society) should flourish and grow. Through reason we bring together the general desire to do good (synderesis) with the natural inclinations we as humans possess and experience. And then we match them up against the eternal law of God, the measure of objective goodness which is also revealed in the natural world.
Veritatis Spendor (1995 Papal encyclical) expresses the natural law in this way, and has changed "worship of God" to "contemplation of beauty". Notice that the Catholic Church has also added a pledge to "refine and develop the riches of the material world", which might be interpreted as a pledge of responsible stewardship. This Roman Catholic papal encyclical summarises the natural law and its origins thus:
"Precisely because of this "truth" the natural law involves universality. Inasmuch as it is inscribed in the rational nature of the person, it makes itself felt to all beings endowed with reason and living in history. In order to perfect himself in his specific order, the person must do good and avoid evil, be concerned for the transmission and preservation of life, refine and develop the riches of the material world, cultivate social life, seek truth, practise good and contemplate beauty. (Veritatis Splendor, 51)