The precepts of natural law are self-evident principles of the practical reason, the reason that is directed to action. In a similar way, the axioms and the demonstrations are self-evident principles of the speculative reason.
What self-evindent means, though?
By self-evident Thomas means an internal property of the propositions, this is,
that the subject and the predicate imply each other. They are self-evident in themselves.
However, In relation to us, these principles are not necessarily self-evident. Some of these
principles are apprehended universally, but others only the wise can gasp.
An example of a self-evident principle that is not gasped by everyone is that
angels are not circumscriptively in a place. This is because angels are not bodies.
What are the first principles that man apprehends?
The first thing that man apprehends universally is the notion of being.
As a consequence, the first self-evident principle of reason is the principle of no contradiction,
since "being" and "not being" cannot be affirmed (or denied at the same time).
The first principle that the practical reason apprehends universally is the notion of good.
This is because good (or something under the appearence of good) is what all things seek after.
Therefore, the first self-evident principle of the practical reason is "seek good and avoid evil"
Classification of the precepts of the natural law
The following precepts belong to the natural law, classified according the order of natural inclinations:
a) precepts following from the nature that man has in common with all substances
b) precepts following from the nature that man has in common with all animals
c) precepts following from the nature that man has in common with himself, what is proper to man.
Thomas' examples of the precepts of natural law according its classification
a) every substance seeks good, and, according to its nature seeks the preservation of its being,
thus, whatever helps the preservation of the human being belong to the natural law.
b) nature has taught to all animals sexual intercourse and education of the offspring,
thus, sexual intercourse and education of the offspring belong to the natural law
c) finally, in accordance to the proper nature of man, that of reason, man has a natural
inclination towards knowing the truth about God, and to live in society. Thus to shun
ignorance and to avoid offending fellow citizens belong to the natural law, regarding
the inclination of man that follows from its rational nature.