What is natural law and its relation to human law and concupiscence? In Treatise on law, questions 91, 94, and 95What are the universal natural...

The essence of natural law is the assertion that there are
objective moral principles which are derived from nature which can be discovered from
reason.  Besides divine law known only to God or given through scripture, Aquinas
determined there was natural law or lex naturalis which is obtained
through reason and lex humana or human law.  Human law is not a
copy of natural law but provides solutions to everyday life because natural law does not
provide all or even most of the solutions for everyday life.  Aquinas also writes that
there is need for human law is to force selfish and evil people to act reasonably.
Aquinas also stated that human law must be virtuous, necessary, useful, and clear and
for the common good.