A moral or aesthetic proposition is like any other proposition, except that there is no objective measuring device for aesthetic or moral propositions. This lack has been addressed recently when a doctor says to a patient "From one to ten how would you describe the severity of the pain?" That is not a physical measuring device and levels of pain can also be measured objectively by signs of fainting or sudden very low blood pressure, pallor and so forth.
Similarly signs of happiness can be objectively measured by behaviours including for instance levels of dopamine.
Aesthetic propositions and moral propositions have in common that they measure the relative strength of a learning opportunity. A "good" work of art embodies a cognitive opportunity. A pain and a pleasure embody an opportunity to learn from conditioned response. A living organism that cannot learn is invariably an organism that has no facility for remembering ; this is a lawlike connection.