I was arguing with someone about the nature of rights, and I said that rights come from reason and can be proved. The response was that certain foundations for proving rights are arbitrary. For example, someone can arbitrarily produce a reason why we have a right to ice cream.
Furthermore, they said that we can only prove we need two things to survive: food and water. Every other right, such as freedom of speech or religion, is arbitrary and that we as a collective only have them because we agreed that life wouldn't be enjoyable if all we did was collect the basic necessities for survival. They said the constitution was put in place, not to recognize rights as natural, but because most people agreed on the idea that rights would make life better for humans.
They also attacked private property rights by stating that if someone, for example, owns the rights to a river, then he can deny water to an individual, and therefore deny him his right to life, as water is needed to survive, thus there should not be property rights for bodies of water. How can one attack these arguments?