Law is judgement between rights and wrongs. Societal law is judgement between rights and wrongs as determined by society. Natural law is judgement between rights and wrongs as determined by nature.
Our minds, which emerge out of nature, have within them faculties of judgement for the sole purpose of making determinations between rights and wrongs; all humans, including irrational ones, regularly use judgement to subjectively determine what is in their best self interest.
This demonstrates that rights and wrongs, as demonstrated by judgement, albeit it subjective ones as determined individually by each human, exist in nature.
Our minds, which, as I said, emerge out of nature, also have within them faculties of reason for the sole purpose of engaging in rational thought; through the use of this rational thought can we determine that it is irrational to form a subjective judgement of rights and wrongs for ourselves, while not extending it to other human beings.
If we judge that it is wrong for us to be killed, it is also wrong to kill others.
This demonstrates that rights and wrongs, as demonstrated by reason, are not just subjective, but objective and universal in nature; that were we to deny the universality of rights and wrongs, we would be animals with judgement, but without reason; savages and beasts.
It is for that reason that any subjective rights that we conceive for ourselves with our natural faculty of judgement, that we can further reason, with our natural faculty of reason, apply objectively to all human beings without infringing upon any other such rights, are in fact universal rights in natural law.
In addition to all of us subjectively judging freedom to be right or a natural right, and by extension of reason, objectively concluding that freedom as a natural right to be universal, nature also grants us the faculty of free thought, by granting us the ability to judge, reason, and form thoughts and opinions in direct contradiction to others who would impose theirs on us.
Society can command you all it wants, through the imposition of societal laws, to think a certain way and to judge rights and wrongs a certain way, but nature has given you a natural right to disregard these unnatural laws; the only way you will consent or comply with these infringements is if, with your own free voluntary judgement and reason, you come to the conclusion that you should.
By the emergence out of nature of your own free thought, not only does it follow that you have a natural right to think freely, but that for you to think freely, there must be a free person with a natural right to be free from the infringement of others to be doing the free thinking.
I naturally think freely, therefore I am naturally free.
It is natural law, not societal law, that grants us natural rights; society merely attempts to codify natural law, while failing to do so when it ends up infringing upon natural rights that emerge out of natural law. It is because natural rights emerge out of natural law that natural rights are inalienable rights; inseparable from nature.
Societal laws which infringe upon natural rights, and are thus in violation of natural law, are an unnatural abomination of humanity.
If you deny or reject this, you are exercising free thought of a free person, an inalienable right granted to you by nature, proving the theorem.