For example, most people who have eaten a mango will say it tastes sweet.
Suppose a foreign person visiting India has never tasted a mango. He has also been advised by his countrymen that bright-looking fruits are usually poisonous. And that people native to India might have a better suited immune system to native fruits, so just because they can eat it doesn't mean he can eat it.
An Indian now offers the foreigner a mango, and claims it tastes sweet.
The foreigner asks for 'PROOF' that it tastes sweet.
The Indian simply says 'Eat it and you'll know'
The foreigner replies 'I will not eat it until you can prove that it tastes sweet ? Only then I will eat it'.
IMO, even if the Indian is well-versed in chemistry/biology and does experiments to show the presence of saccharine molecules in both the mango, and in sugar (an item which the foreigner HAS eaten), the foreigner might still say 'There is no proof that the mere existence of saccharine makes something sweet, especially in combination with other molecules/fibers of the fruit'.
The Indian gives up. Now, the foreigner claims to the world that the Indian's claim of 'Mango is sweet' is unfalsifiable, while conveniently hiding the fact that he has imposed the 'will-not-taste' condition on it.
Have any treatises dealt with the issue of a truth that is subjectively (for me) verifiable, but not objectively (for others) provable ?