Locke believed in the state of nature man is in a society with all of mankind. The law of nature being that of reason. This law stated that no man ought to harm another’s life, liberty or possessions. Locke also believed that knowledge could be broken down into three categories. Intuitive knowledge, demonstrative knowledge and sensitive knowledge. He believed the first two were certain, intuitive knowledge being the most certain. Since Locke had a natural intuition that we ought not to harm the life, liberty or possessions of others he thought this was certain knowledge.
My main issue with the certainty of intuition as a means of creating objective laws is that there are discrepancies between different individuals’ intuitions. The intuition of a violent sociopath might be to take the life of another and if intuitive knowledge is certain then this would create two different mutually exclusive ethics that are both somehow objectively true. One rule would state that it is right to take the life of another while the other would state that taking the life of another is wrong. Logically speaking both these rules cannot be objective laws. This means either some people’s intuitions are the right ones and others are the wrong ones or the more plausible conclusion is that all intuitions are subject to a person’s subconscious values and desires. Another issue with believing that intuition is an infallible source of knowledge is the fact that some people intuitively induce false conclusions about the reality of existence. Racist people for example intuitively induce that some race is superior on all grounds to another. This may be caused by their upbringing or some negative experiences with another race. Either way they might intuitively induce that their race is superior. Scientifically speaking this is both falsifiable and frankly there is no reason to believe that an entire group of people with an arbitrary shared characteristic like skin color is superior on every metric to another. Every individual has a different genetic makeup and an entire life to train a chosen skill set so naturally members of every race will emerge with specialized supperiorities in their respective areas of specialization.
Since we cannot know with certainty through intuition that we have natural rights we are left with a choice of perspective. Either you arbitrarily believe in a metaphysical set of rights that all people are entitled to which might motivate you to implement and protect those rights or you believe that no one automatically is entitled to rights and must come to the conclusion that implementing rights will be for the betterment of every member of society since everyone benefits from an efficient commonwealth. This conclusion can be found when you consider the fact that every person has natural aptitudes and interests and that when every individual is given the freedom to pursue their own natural interests and aptitudes they can more efficiently provide for the commonwealth.
In my view though we do not have natural rights. In a pre-governmental world you have anarchy and animalistic tribalism where the strong dominate over the weak. Nobody is entitled to anything. I think we’ve forgotten our roots and thus take our privilege of basic freedoms for granted. It took a number of strong people to work together to decide that we do in fact need to protect every person’s basic freedoms. This led to it being written into law. Now we can reap the benefits of the work that our founders and former freedom fighters fought for. In a state of nature you have natural selection, but in a state of society you have protection. Now nothings perfect and we’ve got to progress, but we also must feel a deep respect for how far we’ve come. There’s a difference between having the right to something and having a responsibility to something. We have a responsibility to live a just life and progress as a society and as individuals. This responsibility is far from a burden however. The just life provides the greatest fulfillment and so we are privileged by our responsibility.
Locke might argue that in a pre-societal world the people do have their arbitrary rights, but that those rights are just suppressed. I could just as easily arbitrarily argue that I have a right to be a billionaire. Just because society does not protect my right to be a billionaire does not mean my right is being suppressed. A person born into slavery never had rights in the same way I never had a billion dollars. Without ever having gained these things the rights in question are only conceptual. It takes hard work to take those rights. The slaves had to be emancipated before they could be free and have rights. Now society is better for it.