In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when all of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not.
In one sense ethical propositions exist in and of themselves. They can be true, false or even something else entirely without relation to the physical world.
In another sense, however, ethical propositions only exist if there is someone to believe them. In this way, ideas are exposed to the process of evolution in the same way that biological organisms are.