You determine the value, but it's not a whim. It's not something you can make up however you want. If I decide that salt is worth more to me than gold & sell everything I own to buy salt, I will probably discover that in fact there are limits to what I can do with salt and the real well-being or satisfaction I get from the salt is less than I might have hoped. When I discovered this, I will have to sell or otherwise dispose of the salt, which reveals my low valuation upon experience.