The best reason for asserting so bluntly that there are no such rights is precisely the same type as the best reason for asserting that there are no witches: every attempt to give good reasons for believing that there are such rights has failed.
Coincidentally I've been thinking about this for the past few months in the exact same way you have. I haven't thought or found of a secular argument for the self-evident truth of human rights, other than people stating that human rights is a secular religion, which seems iffy and hypocritical to the many anti-religion human rights activists.
The idea of rights are born out of a necessary to field an effective army. Since the rule of thumb is that victory requires one side to have 2-3x the enemy’s numbers, there is a great need for the war-captain to recruit his men and to give them an cohesion. But the risk is high and the men needs a reason to fight. So, the war-captain make an offer of some rights (like a vote) in exchange for loyalty. This was how the common men of Athens won the right to have a say in politics. Athens need a lot of oarsmen for their navy, so the aristocracy gave it to them.
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Coincidentally I've been thinking about this for the past few months in the exact same way you have. I haven't thought or found of a secular argument for the self-evident truth of human rights, other than people stating that human rights is a secular religion, which seems iffy and hypocritical to the many anti-religion human rights activists.
The idea of rights are born out of a necessary to field an effective army. Since the rule of thumb is that victory requires one side to have 2-3x the enemy’s numbers, there is a great need for the war-captain to recruit his men and to give them an cohesion. But the risk is high and the men needs a reason to fight. So, the war-captain make an offer of some rights (like a vote) in exchange for loyalty. This was how the common men of Athens won the right to have a say in politics. Athens need a lot of oarsmen for their navy, so the aristocracy gave it to them.